gitlab-org--gitlab-foss/doc/development/api_graphql_styleguide.md

# GraphQL API style guide

This document outlines the style guide for GitLab's [GraphQL API](../api/graphql/index.md).

## How GitLab implements GraphQL

We use the [GraphQL Ruby gem](https://graphql-ruby.org/) written by [Robert Mosolgo](https://github.com/rmosolgo/).

All GraphQL queries are directed to a single endpoint
([`app/controllers/graphql_controller.rb#execute`](https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fcontrollers%2Fgraphql_controller.rb)),
which is exposed as an API endpoint at `/api/graphql`.

## Deep Dive

In March 2019, Nick Thomas hosted a Deep Dive (GitLab team members only: `https://gitlab.com/gitlab-org/create-stage/issues/1`)
on GitLab's [GraphQL API](../api/graphql/index.md) to share his domain specific knowledge
with anyone who may work in this part of the code base in the future. You can find the
[recording on YouTube](https://www.youtube.com/watch?v=-9L_1MWrjkg), and the slides on
[Google Slides](https://docs.google.com/presentation/d/1qOTxpkTdHIp1CRjuTvO-aXg0_rUtzE3ETfLUdnBB5uQ/edit)
and in [PDF](https://gitlab.com/gitlab-org/create-stage/uploads/8e78ea7f326b2ef649e7d7d569c26d56/GraphQL_Deep_Dive__Create_.pdf).
Everything covered in this deep dive was accurate as of GitLab 11.9, and while specific
details may have changed since then, it should still serve as a good introduction.

## GraphiQL

GraphiQL is an interactive GraphQL API explorer where you can play around with existing queries.
You can access it in any GitLab environment on `https://<your-gitlab-site.com>/-/graphql-explorer`.
For example, the one for [GitLab.com](https://gitlab.com/-/graphql-explorer).

## Authentication

Authentication happens through the `GraphqlController`, right now this
uses the same authentication as the Rails application. So the session
can be shared.

It is also possible to add a `private_token` to the querystring, or
add a `HTTP_PRIVATE_TOKEN` header.

## Types

We use a code-first schema, and we declare what type everything is in Ruby.

For example, `app/graphql/types/issue_type.rb`:

```ruby
graphql_name 'Issue'

field :iid, GraphQL::ID_TYPE, null: true
field :title, GraphQL::STRING_TYPE, null: true

# we also have a method here that we've defined, that extends `field`
markdown_field :title_html, null: true
field :description, GraphQL::STRING_TYPE, null: true
markdown_field :description_html, null: true
```

We give each type a name (in this case `Issue`).

The `iid`, `title` and `description` are _scalar_ GraphQL types.
`iid` is a `GraphQL::ID_TYPE`, a special string type that signifies a unique ID.
`title` and `description` are regular `GraphQL::STRING_TYPE` types.

When exposing a model through the GraphQL API, we do so by creating a
new type in `app/graphql/types`. You can also declare custom GraphQL data types
for scalar data types (e.g. `TimeType`).

When exposing properties in a type, make sure to keep the logic inside
the definition as minimal as possible. Instead, consider moving any
logic into a presenter:

```ruby
class Types::MergeRequestType < BaseObject
  present_using MergeRequestPresenter

  name 'MergeRequest'
end
```

An existing presenter could be used, but it is also possible to create
a new presenter specifically for GraphQL.

The presenter is initialized using the object resolved by a field, and
the context.

### Nullable fields

GraphQL allows fields to be "nullable" or "non-nullable". The former means
that `null` may be returned instead of a value of the specified type. **In
general**, you should prefer using nullable fields to non-nullable ones, for
the following reasons:

- It's common for data to switch from required to not-required, and back again
- Even when there is no prospect of a field becoming optional, it may not be **available** at query time
  - For instance, the `content` of a blob may need to be looked up from Gitaly
  - If the `content` is nullable, we can return a **partial** response, instead of failing the whole query
- Changing from a non-nullable field to a nullable field is difficult with a versionless schema

Non-nullable fields should only be used when a field is required, very unlikely
to become optional in the future, and very easy to calculate. An example would
be `id` fields.

Further reading:

- [GraphQL Best Practices Guide](https://graphql.org/learn/best-practices/#nullability)
- [Using nullability in GraphQL](https://www.apollographql.com/blog/using-nullability-in-graphql-2254f84c4ed7)

### Exposing Global IDs

When exposing an `ID` field on a type, we will by default try to
expose a global ID by calling `to_global_id` on the resource being
rendered.

To override this behavior, you can implement an `id` method on the
type for which you are exposing an ID. Please make sure that when
exposing a `GraphQL::ID_TYPE` using a custom method that it is
globally unique.

The records that are exposing a `full_path` as an `ID_TYPE` are one of
these exceptions. Since the full path is a unique identifier for a
`Project` or `Namespace`.

### Connection Types

GraphQL uses [cursor based
pagination](https://graphql.org/learn/pagination/#pagination-and-edges)
to expose collections of items. This provides the clients with a lot
of flexibility while also allowing the backend to use different
pagination models.

To expose a collection of resources we can use a connection type. This wraps the array with default pagination fields. For example a query for project-pipelines could look like this:

```graphql
query($project_path: ID!) {
  project(fullPath: $project_path) {
    pipelines(first: 2) {
      pageInfo {
        hasNextPage
        hasPreviousPage
      }
      edges {
        cursor
        node {
          id
          status
        }
      }
    }
  }
}
```

This would return the first 2 pipelines of a project and related
pagination information, ordered by descending ID. The returned data would
look like this:

```json
{
  "data": {
    "project": {
      "pipelines": {
        "pageInfo": {
          "hasNextPage": true,
          "hasPreviousPage": false
        },
        "edges": [
          {
            "cursor": "Nzc=",
            "node": {
              "id": "gid://gitlab/Pipeline/77",
              "status": "FAILED"
            }
          },
          {
            "cursor": "Njc=",
            "node": {
              "id": "gid://gitlab/Pipeline/67",
              "status": "FAILED"
            }
          }
        ]
      }
    }
  }
}
```

To get the next page, the cursor of the last known element could be
passed:

```graphql
query($project_path: ID!) {
  project(fullPath: $project_path) {
    pipelines(first: 2, after: "Njc=") {
      pageInfo {
        hasNextPage
        hasPreviousPage
      }
      edges {
        cursor
        node {
          id
          status
        }
      }
    }
  }
}
```

To ensure that we get consistent ordering, we will append an ordering on the primary
key, in descending order. This is usually `id`, so basically we will add `order(id: :desc)`
to the end of the relation. A primary key _must_ be available on the underlying table.

#### Shortcut fields

Sometimes it can seem easy to implement a "shortcut field", having the resolver return the first of a collection if no parameters are passed.
These "shortcut fields" are discouraged because they create maintenance overhead.
They need to be kept in sync with their canonical field, and deprecated or modified if their canonical field changes.
Use the functionality the framework provides unless there is a compelling reason to do otherwise.

For example, instead of `latest_pipeline`, use `pipelines(last: 1)`.

### Exposing permissions for a type

To expose permissions the current user has on a resource, you can call
the `expose_permissions` passing in a separate type representing the
permissions for the resource.

For example:

```ruby
module Types
  class MergeRequestType < BaseObject
    expose_permissions Types::MergeRequestPermissionsType
  end
end
```

The permission type inherits from `BasePermissionType` which includes
some helper methods, that allow exposing permissions as non-nullable
booleans:

```ruby
class MergeRequestPermissionsType < BasePermissionType
  present_using MergeRequestPresenter

  graphql_name 'MergeRequestPermissions'

  abilities :admin_merge_request, :update_merge_request, :create_note

  ability_field :resolve_note,
                description: 'Indicates the user can resolve discussions on the merge request'
  permission_field :push_to_source_branch, method: :can_push_to_source_branch?
end
```

- **`permission_field`**: Will act the same as `graphql-ruby`'s
  `field` method but setting a default description and type and making
  them non-nullable. These options can still be overridden by adding
  them as arguments.
- **`ability_field`**: Expose an ability defined in our policies. This
  behaves the same way as `permission_field` and the same
  arguments can be overridden.
- **`abilities`**: Allows exposing several abilities defined in our
  policies at once. The fields for these will all have be non-nullable
  booleans with a default description.

## Feature flags

Developers can add [feature flags](../development/feature_flags/index.md) to GraphQL
fields in the following ways:

- Add the `feature_flag` property to a field. This will allow the field to be _hidden_
  from the GraphQL schema when the flag is disabled.
- Toggle the return value when resolving the field.

You can refer to these guidelines to decide which approach to use:

- If your field is experimental, and its name or type is subject to
  change, use the `feature_flag` property.
- If your field is stable and its definition will not change, even after the flag is
  removed, toggle the return value of the field instead. Note that
  [all fields should be nullable](#nullable-fields) anyway.

### `feature_flag` property

The `feature_flag` property allows you to toggle the field's
[visibility](https://graphql-ruby.org/authorization/visibility.html)
within the GraphQL schema. This will remove the field from the schema
when the flag is disabled.

A description is [appended](https://gitlab.com/gitlab-org/gitlab/-/blob/497b556/app/graphql/types/base_field.rb#L44-53)
to the field indicating that it is behind a feature flag.

CAUTION: **Caution:**
If a client queries for the field when the feature flag is disabled, the query will
fail. Consider this when toggling the visibility of the feature on or off on
production.

The `feature_flag` property does not allow the use of
[feature gates based on actors](../development/feature_flags/development.md).
This means that the feature flag cannot be toggled only for particular
projects, groups, or users, but instead can only be toggled globally for
everyone.

Example:

```ruby
field :test_field, type: GraphQL::STRING_TYPE,
      null: true,
      description: 'Some test field',
      feature_flag: :my_feature_flag
```

### Toggle the value of a field

This method of using feature flags for fields is to toggle the
return value of the field. This can be done in the resolver, in the
type, or even in a model method, depending on your preference and
situation.

When applying a feature flag to toggle the value of a field, the
`description` of the field must:

- State that the value of the field can be toggled by a feature flag.
- Name the feature flag.
- State what the field will return when the feature flag is disabled (or
  enabled, if more appropriate).

Example:

```ruby
field :foo, GraphQL::STRING_TYPE,
      null: true,
      description: 'Some test field. Will always return `null`' \
                   'if `my_feature_flag` feature flag is disabled'

def foo
  object.foo unless Feature.enabled?(:my_feature_flag, object)
end
```

## Deprecating fields

GitLab's GraphQL API is versionless, which means we maintain backwards
compatibility with older versions of the API with every change. Rather
than removing a field, we need to _deprecate_ the field instead. In
future, GitLab
[may remove deprecated fields](https://gitlab.com/gitlab-org/gitlab/-/issues/32292).

Fields are deprecated using the `deprecated` property. The value
of the property is a `Hash` of:

- `reason` - Reason for the deprecation.
- `milestone` - Milestone that the field was deprecated.

Example:

```ruby
field :token, GraphQL::STRING_TYPE, null: true,
      deprecated: { reason: 'Login via token has been removed', milestone: '10.0' },
      description: 'Token for login'
```

The original `description:` of the field should be maintained, and should
_not_ be updated to mention the deprecation.

### Deprecation reason style guide

Where the reason for deprecation is due to the field being replaced
with another field, the `reason` must be:

```plaintext
Use `otherFieldName`
```

Example:

```ruby
field :designs, ::Types::DesignManagement::DesignCollectionType, null: true,
      deprecated: { reason: 'Use `designCollection`', milestone: '10.0' },
      description: 'The designs associated with this issue',
```

If the field is not being replaced by another field, a descriptive
deprecation `reason` should be given.

## Enums

GitLab GraphQL enums are defined in `app/graphql/types`. When defining new enums, the
following rules apply:

- Values must be uppercase.
- Class names must end with the string `Enum`.
- The `graphql_name` must not contain the string `Enum`.

For example:

```ruby
module Types
  class TrafficLightStateEnum < BaseEnum
    graphql_name 'TrafficLightState'
    description 'State of a traffic light'

    value 'RED', description: 'Drivers must stop'
    value 'YELLOW', description: 'Drivers must stop when it is safe to'
    value 'GREEN', description: 'Drivers can start or keep driving'
  end
end
```

If the enum will be used for a class property in Ruby that is not an uppercase string,
you can provide a `value:` option that will adapt the uppercase value.

In the following example:

- GraphQL inputs of `OPENED` will be converted to `'opened'`.
- Ruby values of `'opened'` will be converted to `"OPENED"` in GraphQL responses.

```ruby
module Types
  class EpicStateEnum < BaseEnum
    graphql_name 'EpicState'
    description 'State of a GitLab epic'

    value 'OPENED', value: 'opened', description: 'An open Epic'
    value 'CLOSED', value: 'closed', description: 'An closed Epic'
  end
end
```

## JSON

When data to be returned by GraphQL is stored as
[JSON](migration_style_guide.md#storing-json-in-database), we should continue to use
GraphQL types whenever possible. Avoid using the `GraphQL::Types::JSON` type unless
the JSON data returned is _truly_ unstructured.

If the structure of the JSON data varies, but will be one of a set of known possible
structures, use a
[union](https://graphql-ruby.org/type_definitions/unions.html).
An example of the use of a union for this purpose is
[!30129](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/30129).

Field names can be mapped to hash data keys using the `hash_key:` keyword if needed.

For example, given the following simple JSON data:

```json
{
  "title": "My chart",
  "data": [
    { "x": 0, "y": 1 },
    { "x": 1, "y": 1 },
    { "x": 2, "y": 2 }
  ]
}
```

We can use GraphQL types like this:

```ruby
module Types
  class ChartType < BaseObject
    field :title, GraphQL::STRING_TYPE, null: true, description: 'Title of the chart'
    field :data, [Types::ChartDatumType], null: true, description: 'Data of the chart'
  end
end

module Types
  class ChartDatumType < BaseObject
    field :x, GraphQL::INT_TYPE, null: true, description: 'X-axis value of the chart datum'
    field :y, GraphQL::INT_TYPE, null: true, description: 'Y-axis value of the chart datum'
  end
end
```

## Descriptions

All fields and arguments
[must have descriptions](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/16438).

A description of a field or argument is given using the `description:`
keyword. For example:

```ruby
field :id, GraphQL::ID_TYPE, description: 'ID of the resource'
```

Descriptions of fields and arguments are viewable to users through:

- The [GraphiQL explorer](#graphiql).
- The [static GraphQL API reference](../api/graphql/#reference).

### Description style guide

To ensure consistency, the following should be followed whenever adding or updating
descriptions:

- Mention the name of the resource in the description. Example:
  `'Labels of the issue'` (issue being the resource).
- Use `"{x} of the {y}"` where possible. Example: `'Title of the issue'`.
  Do not start descriptions with `The`.
- Descriptions of `GraphQL::BOOLEAN_TYPE` fields should answer the question: "What does
  this field do?". Example: `'Indicates project has a Git repository'`.
- Always include the word `"timestamp"` when describing an argument or
  field of type `Types::TimeType`. This lets the reader know that the
  format of the property will be `Time`, rather than just `Date`.
- No `.` at end of strings.

Example:

```ruby
field :id, GraphQL::ID_TYPE, description: 'ID of the Issue'
field :confidential, GraphQL::BOOLEAN_TYPE, description: 'Indicates the issue is confidential'
field :closed_at, Types::TimeType, description: 'Timestamp of when the issue was closed'
```

### `copy_field_description` helper

Sometimes we want to ensure that two descriptions will always be identical.
For example, to keep a type field description the same as a mutation argument
when they both represent the same property.

Instead of supplying a description, we can use the `copy_field_description` helper,
passing it the type, and field name to copy the description of.

Example:

```ruby
argument :title, GraphQL::STRING_TYPE,
          required: false,
          description: copy_field_description(Types::MergeRequestType, :title)
```

## Authorization

Authorizations can be applied to both types and fields using the same
abilities as in the Rails app.

If the:

- Currently authenticated user fails the authorization, the authorized
  resource will be returned as `null`.
- Resource is part of a collection, the collection will be filtered to
  exclude the objects that the user's authorization checks failed against.

Also see [authorizing resources in a mutation](#authorizing-resources).

TIP: **Tip:**
Try to load only what the currently authenticated user is allowed to
view with our existing finders first, without relying on authorization
to filter the records. This minimizes database queries and unnecessary
authorization checks of the loaded records.

### Type authorization

Authorize a type by passing an ability to the `authorize` method. All
fields with the same type will be authorized by checking that the
currently authenticated user has the required ability.

For example, the following authorization ensures that the currently
authenticated user can only see projects that they have the
`read_project` ability for (so long as the project is returned in a
field that uses `Types::ProjectType`):

```ruby
module Types
  class ProjectType < BaseObject
    authorize :read_project
  end
end
```

You can also authorize against multiple abilities, in which case all of
the ability checks must pass.

For example, the following authorization ensures that the currently
authenticated user must have `read_project` and `another_ability`
abilities to see a project:

```ruby
module Types
  class ProjectType < BaseObject
    authorize [:read_project, :another_ability]
  end
end
```

### Field authorization

Fields can be authorized with the `authorize` option.

For example, the following authorization ensures that the currently
authenticated user must have the `owner_access` ability to see the
project:

```ruby
module Types
  class MyType < BaseObject
    field :project, Types::ProjectType, null: true, resolver: Resolvers::ProjectResolver, authorize: :owner_access
  end
end
```

Fields can also be authorized against multiple abilities, in which case
all of ability checks must pass. **Note:** This requires explicitly
passing a block to `field`:

```ruby
module Types
  class MyType < BaseObject
    field :project, Types::ProjectType, null: true, resolver: Resolvers::ProjectResolver do
      authorize [:owner_access, :another_ability]
    end
  end
end
```

NOTE: **Note:**
If the field's type already [has a particular
authorization](#type-authorization) then there is no need to add that
same authorization to the field.

### Type and Field authorizations together

Authorizations are cumulative, so where authorizations are defined on
a field, and also on the field's type, then the currently authenticated
user would need to pass all ability checks.

In the following simplified example the currently authenticated user
would need both `first_permission` and `second_permission` abilities in
order to see the author of the issue.

```ruby
class UserType
  authorize :first_permission
end
```

```ruby
class IssueType
  field :author, UserType, authorize: :second_permission
end
```

## Resolvers

We define how the application serves the response using _resolvers_
stored in the `app/graphql/resolvers` directory.
The resolver provides the actual implementation logic for retrieving
the objects in question.

To find objects to display in a field, we can add resolvers to
`app/graphql/resolvers`.

Arguments can be defined within the resolver, those arguments will be
made available to the fields using the resolver. When exposing a model
that had an internal ID (`iid`), prefer using that in combination with
the namespace path as arguments in a resolver over a database
ID. Otherwise use a [globally unique ID](#exposing-global-ids).

We already have a `FullPathLoader` that can be included in other
resolvers to quickly find Projects and Namespaces which will have a
lot of dependent objects.

To limit the amount of queries performed, we can use `BatchLoader`.

### Correct use of `Resolver#ready?`

Resolvers have two public API methods as part of the framework: `#ready?(**args)` and `#resolve(**args)`.
We can use `#ready?` to perform set-up, validation or early-return without invoking `#resolve`.

Good reasons to use `#ready?` include:

- validating mutually exclusive arguments (see [validating arguments](#validating-arguments))
- Returning `Relation.none` if we know before-hand that no results are possible
- Performing setup such as initializing instance variables (although consider lazily initialized methods for this)

Implementations of [`Resolver#ready?(**args)`](https://graphql-ruby.org/api-doc/1.10.9/GraphQL/Schema/Resolver#ready%3F-instance_method) should
return `(Boolean, early_return_data)` as follows:

```ruby
def ready?(**args)
  [false, 'have this instead']
end
```

For this reason, whenever you call a resolver (mainly in tests - as framework
abstractions Resolvers should not be considered re-usable, finders are to be
preferred), remember to call the `ready?` method and check the boolean flag
before calling `resolve`! An example can be seen in our [`GraphQLHelpers`](https://gitlab.com/gitlab-org/gitlab/-/blob/2d395f32d2efbb713f7bc861f96147a2a67e92f2/spec/support/helpers/graphql_helpers.rb#L20-27).

### Look-Ahead

The full query is known in advance during execution, which means we can make use
of [lookahead](https://graphql-ruby.org/queries/lookahead.html) to optimize our
queries, and batch load associations we know we will need. Consider adding
lookahead support in your resolvers to avoid `N+1` performance issues.

To enable support for common lookahead use-cases (pre-loading associations when
child fields are requested), you can
include [`LooksAhead`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/graphql/resolvers/concerns/looks_ahead.rb). For example:

```ruby
# Assuming a model `MyThing` with attributes `[child_attribute, other_attribute, nested]`,
# where nested has an attribute named `included_attribute`.
class MyThingResolver < BaseResolver
  include LooksAhead

  # Rather than defining `resolve(**args)`, we implement: `resolve_with_lookahead(**args)`
  def resolve_with_lookahead(**args)
    apply_lookahead(MyThingFinder.new(current_user).execute)
  end

  # We list things that should always be preloaded:
  # For example, if child_attribute is always needed (during authorization
  # perhaps), then we can include it here.
  def unconditional_includes
    [:child_attribute]
  end

  # We list things that should be included if a certain field is selected:
  def preloads
    {
        field_one: [:other_attribute],
        field_two: [{ nested: [:included_attribute] }]
    }
  end
end
```

The final thing that is needed is that every field that uses this resolver needs
to advertise the need for lookahead:

```ruby
  # in ParentType
  field :my_things, MyThingType.connection_type, null: true,
        extras: [:lookahead], # Necessary
        resolver: MyThingResolver,
        description: 'My things'
```

For an example of real world use, please
see [`ResolvesMergeRequests`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/graphql/resolvers/concerns/resolves_merge_requests.rb).

## Mutations

Mutations are used to change any stored values, or to trigger
actions. In the same way a GET-request should not modify data, we
cannot modify data in a regular GraphQL-query. We can however in a
mutation.

To find objects for a mutation, arguments need to be specified. As with
[resolvers](#resolvers), prefer using internal ID or, if needed, a
global ID rather than the database ID.

### Building Mutations

Mutations live in `app/graphql/mutations` ideally grouped per
resources they are mutating, similar to our services. They should
inherit `Mutations::BaseMutation`. The fields defined on the mutation
will be returned as the result of the mutation.

### Naming conventions

Each mutation must define a `graphql_name`, which is the name of the mutation in the GraphQL schema.

Example:

```ruby
class UserUpdateMutation < BaseMutation
  graphql_name 'UserUpdate'
end
```

Our GraphQL mutation names are historically inconsistent, but new mutation names should follow the
convention `'{Resource}{Action}'` or `'{Resource}{Action}{Attribute}'`.

Mutations that **create** new resources should use the verb `Create`.

Example:

- `CommitCreate`

Mutations that **update** data should use:

- The verb `Update`.
- A domain-specific verb like `Set`, `Add`, or `Toggle` if more appropriate.

Examples:

- `EpicTreeReorder`
- `IssueSetWeight`
- `IssueUpdate`
- `TodoMarkDone`

Mutations that **remove** data should use:

- The verb `Delete` rather than `Destroy`.
- A domain-specific verb like `Remove` if more appropriate.

Examples:

- `AwardEmojiRemove`
- `NoteDelete`

If you need advice for mutation naming, canvass the Slack `#graphql` channel for feedback.

### Arguments

Arguments required by the mutation can be defined as arguments
required for a field. These will be wrapped up in an input type for
the mutation. For example, the `Mutations::MergeRequests::SetWip`
with GraphQL-name `MergeRequestSetWip` defines these arguments:

```ruby
argument :project_path, GraphQL::ID_TYPE,
         required: true,
         description: "The project the merge request to mutate is in"

argument :iid, GraphQL::STRING_TYPE,
         required: true,
         description: "The iid of the merge request to mutate"

argument :wip,
         GraphQL::BOOLEAN_TYPE,
         required: false,
         description: <<~DESC
                      Whether or not to set the merge request as a WIP.
                      If not passed, the value will be toggled.
                      DESC
```

This would automatically generate an input type called
`MergeRequestSetWipInput` with the 3 arguments we specified and the
`clientMutationId`.

These arguments are then passed to the `resolve` method of a mutation
as keyword arguments.

### Fields

In the most common situations, a mutation would return 2 fields:

- The resource being modified
- A list of errors explaining why the action could not be
  performed. If the mutation succeeded, this list would be empty.

By inheriting any new mutations from `Mutations::BaseMutation` the
`errors` field is automatically added. A `clientMutationId` field is
also added, this can be used by the client to identify the result of a
single mutation when multiple are performed within a single request.

### The `resolve` method

The `resolve` method receives the mutation's arguments as keyword arguments.
From here, we can call the service that will modify the resource.

The `resolve` method should then return a hash with the same field
names as defined on the mutation including an `errors` array. For example,
the `Mutations::MergeRequests::SetWip` defines a `merge_request`
field:

```ruby
field :merge_request,
      Types::MergeRequestType,
      null: true,
      description: "The merge request after mutation"
```

This means that the hash returned from `resolve` in this mutation
should look like this:

```ruby
{
  # The merge request modified, this will be wrapped in the type
  # defined on the field
  merge_request: merge_request,
  # An array of strings if the mutation failed after authorization.
  # The `errors_on_object` helper collects `errors.full_messages`
  errors: errors_on_object(merge_request)
}
```

### Mounting the mutation

To make the mutation available it must be defined on the mutation
type that lives in `graphql/types/mutation_types`. The
`mount_mutation` helper method will define a field based on the
GraphQL-name of the mutation:

```ruby
module Types
  class MutationType < BaseObject
    include Gitlab::Graphql::MountMutation

    graphql_name "Mutation"

    mount_mutation Mutations::MergeRequests::SetWip
  end
end
```

Will generate a field called `mergeRequestSetWip` that
`Mutations::MergeRequests::SetWip` to be resolved.

### Authorizing resources

To authorize resources inside a mutation, we first provide the required
 abilities on the mutation like this:

```ruby
module Mutations
  module MergeRequests
    class SetWip < Base
      graphql_name 'MergeRequestSetWip'

      authorize :update_merge_request
    end
  end
end
```

We can then call `authorize!` in the `resolve` method, passing in the resource we
want to validate the abilities for.

Alternatively, we can add a `find_object` method that will load the
object on the mutation. This would allow you to use the
`authorized_find!` helper method.

When a user is not allowed to perform the action, or an object is not
found, we should raise a
`Gitlab::Graphql::Errors::ResourceNotAvailable` error. Which will be
correctly rendered to the clients.

### Errors in mutations

We encourage following the practice of [errors as
data](https://graphql-ruby.org/mutations/mutation_errors) for mutations, which
distinguishes errors by who they are relevant to, defined by who can deal with
them.

Key points:

- All mutation responses have an `errors` field. This should be populated on
  failure, and may be populated on success.
- Consider who needs to see the error: the **user** or the **developer**.
- Clients should always request the `errors` field when performing mutations.
- Errors may be reported to users either at `$root.errors` (top-level error) or at
  `$root.data.mutationName.errors` (mutation errors). The location depends on what kind of error
  this is, and what information it holds.

Consider an example mutation `doTheThing` that returns a response with
two fields: `errors: [String]`, and `thing: ThingType`. The specific nature of
the `thing` itself is irrelevant to these examples, as we are considering the
errors.

There are three states a mutation response can be in:

- [Success](#success)
- [Failure (relevant to the user)](#failure-relevant-to-the-user)
- [Failure (irrelevant to the user)](#failure-irrelevant-to-the-user)

#### Success

In the happy path, errors *may* be returned, along with the anticipated payload, but
if everything was successful, then `errors` should be an empty array, since
there are no problems we need to inform the user of.

```javascript
{
  data: {
    doTheThing: {
      errors: [] // if successful, this array will generally be empty.
      thing: { .. }
    }
  }
}
```

#### Failure (relevant to the user)

An error that affects the **user** occurred. We refer to these as _mutation errors_. In
this case there is typically no `thing` to return:

```javascript
{
  data: {
    doTheThing: {
      errors: ["you cannot touch the thing"],
      thing: null
    }
  }
}
```

Examples of this include:

- Model validation errors: the user may need to change the inputs.
- Permission errors: the user needs to know they cannot do this, they may need to request permission or sign in.
- Problems with application state that prevent the user's action, for example: merge conflicts, the resource was locked, and so on.

Ideally, we should prevent the user from getting this far, but if they do, they
need to be told what is wrong, so they understand the reason for the failure and
what they can do to achieve their intent, even if that is as simple as retrying the
request.

It is possible to return *recoverable* errors alongside mutation data. For example, if
a user uploads 10 files and 3 of them fail and the rest succeed, the errors for the
failures can be made available to the user, alongside the information about
the successes.

#### Failure (irrelevant to the user)

One or more *non-recoverable* errors can be returned at the _top level_. These
are things over which the **user** has little to no control, and should mainly
be system or programming problems, that a **developer** needs to know about.
In this case there is no `data`:

```javascript
{
  errors: [
    {"message": "argument error: expected an integer, got null"},
  ]
}
```

This is the result of raising an error during the mutation. In our implementation,
the messages of argument errors and validation errors are returned to the client, and all other
`StandardError` instances are caught, logged and presented to the client with the message set to `"Internal server error"`.
See [`GraphqlController`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/controllers/graphql_controller.rb) for details.

These represent programming errors, such as:

- A GraphQL syntax error, where an `Int` was passed instead of a `String`, or a required argument was not present.
- Errors in our schema, such as being unable to provide a value for a non-nullable field.
- System errors: for example, a Git storage exception, or database unavailability.

The user should not be able to cause such errors in regular usage. This category
of errors should be treated as internal, and not shown to the user in specific
detail.

We need to inform the user when the mutation fails, but we do not need to
tell them why, since they cannot have caused it, and nothing they can do will
fix it, although we may offer to retry the mutation.

#### Categorizing errors

When we write mutations, we need to be conscious about which of
these two categories an error state falls into (and communicate about this with
frontend developers to verify our assumptions). This means distinguishing the
needs of the _user_ from the needs of the _client_.

> _Never catch an error unless the user needs to know about it._

If the user does need to know about it, communicate with frontend developers
to make sure the error information we are passing back is useful.

See also the [frontend GraphQL guide](../development/fe_guide/graphql.md#handling-errors).

### Aliasing and deprecating mutations

The `#mount_aliased_mutation` helper allows us to alias a mutation as
another name within `MutationType`.

For example, to alias a mutation called `FooMutation` as `BarMutation`:

```ruby
mount_aliased_mutation 'BarMutation', Mutations::FooMutation
```

This allows us to rename a mutation and continue to support the old name,
when coupled with the [`deprecated`](#deprecating-fields) argument.

Example:

```ruby
mount_aliased_mutation 'UpdateFoo',
                        Mutations::Foo::Update,
                        deprecated: { reason: 'Use fooUpdate', milestone: '13.2' }
```

Deprecated mutations should be added to `Types::DeprecatedMutations` and
tested for within the unit test of `Types::MutationType`. The merge request
[!34798](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34798)
can be referred to as an example of this, including the method of testing
deprecated aliased mutations.

## Validating arguments

For validations of single arguments, use the
[`prepare` option](https://github.com/rmosolgo/graphql-ruby/blob/master/guides/fields/arguments.md)
as normal.

Sometimes a mutation or resolver may accept a number of optional
arguments, but we still want to validate that at least one of the optional
arguments is provided. In this situation, consider using the `#ready?`
method within your mutation or resolver to provide the validation. The
`#ready?` method will be called before any work is done within the
`#resolve` method.

Example:

```ruby
def ready?(**args)
  if args.values_at(:body, :position).compact.blank?
    raise Gitlab::Graphql::Errors::ArgumentError,
          'body or position arguments are required'
  end

  # Always remember to call `#super`
  super
end
```

In the future this may be able to be done using `InputUnions` if
[this RFC](https://github.com/graphql/graphql-spec/blob/master/rfcs/InputUnion.md)
is merged.

## GitLab's custom scalars

### `Types::TimeType`

[`Types::TimeType`](https://gitlab.com/gitlab-org/gitlab/blob/master/app%2Fgraphql%2Ftypes%2Ftime_type.rb)
must be used as the type for all fields and arguments that deal with Ruby
`Time` and `DateTime` objects.

The type is
[a custom scalar](https://github.com/rmosolgo/graphql-ruby/blob/master/guides/type_definitions/scalars.md#custom-scalars)
that:

- Converts Ruby's `Time` and `DateTime` objects into standardized
  ISO-8601 formatted strings, when used as the type for our GraphQL fields.
- Converts ISO-8601 formatted time strings into Ruby `Time` objects,
  when used as the type for our GraphQL arguments.

This allows our GraphQL API to have a standardized way that it presents time
and handles time inputs.

Example:

```ruby
field :created_at, Types::TimeType, null: true, description: 'Timestamp of when the issue was created'
```

## Testing

_full stack_ tests for a graphql query or mutation live in
`spec/requests/api/graphql`.

When adding a query, the `a working graphql query` shared example can
be used to test if the query renders valid results.

Using the `GraphqlHelpers#all_graphql_fields_for`-helper, a query
including all available fields can be constructed. This makes it easy
to add a test rendering all possible fields for a query.

To test GraphQL mutation requests, `GraphqlHelpers` provides 2
helpers: `graphql_mutation` which takes the name of the mutation, and
a hash with the input for the mutation. This will return a struct with
a mutation query, and prepared variables.

This struct can then be passed to the `post_graphql_mutation` helper,
that will post the request with the correct parameters, like a GraphQL
client would do.

To access the response of a mutation, the `graphql_mutation_response`
helper is available.

Using these helpers, we can build specs like this:

```ruby
let(:mutation) do
  graphql_mutation(
    :merge_request_set_wip,
    project_path: 'gitlab-org/gitlab-foss',
    iid: '1',
    wip: true
  )
end

it 'returns a successful response' do
   post_graphql_mutation(mutation, current_user: user)

   expect(response).to have_gitlab_http_status(:success)
   expect(graphql_mutation_response(:merge_request_set_wip)['errors']).to be_empty
end
```

## Notes about Query flow and GraphQL infrastructure

GitLab's GraphQL infrastructure can be found in `lib/gitlab/graphql`.

[Instrumentation](https://graphql-ruby.org/queries/instrumentation.html) is functionality
that wraps around a query being executed. It is implemented as a module that uses the `Instrumentation` class.

Example: `Present`

```ruby
module Gitlab
  module Graphql
    module Present
      #... some code above...

      def self.use(schema_definition)
        schema_definition.instrument(:field, ::Gitlab::Graphql::Present::Instrumentation.new)
      end
    end
  end
end
```

A [Query Analyzer](https://graphql-ruby.org/queries/ast_analysis.html#analyzer-api) contains a series
of callbacks to validate queries before they are executed. Each field can pass through
the analyzer, and the final value is also available to you.

[Multiplex queries](https://graphql-ruby.org/queries/multiplex.html) enable
multiple queries to be sent in a single request. This reduces the number of requests sent to the server.
(there are custom Multiplex Query Analyzers and Multiplex Instrumentation provided by GraphQL Ruby).

### Query limits

Queries and mutations are limited by depth, complexity, and recursion
to protect server resources from overly ambitious or malicious queries.
These values can be set as defaults and overridden in specific queries as needed.
The complexity values can be set per object as well, and the final query complexity is
evaluated based on how many objects are being returned. This is useful
for objects that are expensive (e.g. requiring Gitaly calls).

For example, a conditional complexity method in a resolver:

```ruby
def self.resolver_complexity(args, child_complexity:)
  complexity = super
  complexity += 2 if args[:labelName]

  complexity
end
```

More about complexity:
[GraphQL Ruby documentation](https://graphql-ruby.org/queries/complexity_and_depth.html).

## Documentation and Schema

Our schema is located at `app/graphql/gitlab_schema.rb`.
See the [schema reference](../api/graphql/reference/index.md) for details.

This generated GraphQL documentation needs to be updated when the schema changes.
For information on generating GraphQL documentation and schema files, see
[updating the schema documentation](rake_tasks.md#update-graphql-documentation-and-schema-definitions).

To help our readers, you should also add a new page to our [GraphQL API](../api/graphql/index.md) documentation.
For guidance, see the [GraphQL API](documentation/styleguide.md#graphql-api) section
of our documentation style guide.