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

GraphQL API

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.

Authorization

Fields can be authorized using the same abilities used in the Rails app. This can be done using the authorize helper:

module Types
  class QueryType < BaseObject
    graphql_name 'Query'

    field :project, Types::ProjectType, null: true, resolver: Resolvers::ProjectResolver do
      authorize :read_project
    end
  end

The object found by the resolve call is used for authorization.

This works for authorizing a single record, for authorizing collections, we should only load what the currently authenticated user is allowed to view. Preferably we use our existing finders for that.

Types

When exposing a model through the GraphQL API, we do so by creating a new type in app/graphql/types.

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:

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.

Connection Types

GraphQL uses cursor based pagination 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:

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 info., ordered by descending ID. The returned data would look like this:

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

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

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

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:

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:

class MergeRequestPermissionsType < BasePermissionType
  present_using MergeRequestPresenter

  graphql_name 'MergeRequestPermissions'

  abilities :admin_merge_request, :update_merge_request, :create_note

  ability_field :resolve_note,
                description: 'Whether or not the user can resolve disussions 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 takes 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.

Resolvers

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.

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

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

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.

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.

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.

Always provide a consistent GraphQL-name to the mutation, this name is used to generate the input types and the field the mutation is mounted on. The name should look like <Resource being modified><Mutation class name>, for example the Mutations::MergeRequests::SetWip mutation has GraphQL name MergeRequestSetWip.

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:

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

argument :iid, GraphQL::ID_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. 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 and an errors array. For example, the Mutations::MergeRequests::SetWip defines a merge_request field:

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:

{
  # The merge request modified, this will be wrapped in the type
  # defined on the field
  merge_request: merge_request,
  # An array if strings if the mutation failed after authorization
  errors: merge_request.errors.full_messages
}

To make the mutation available it should 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:

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 can include the Gitlab::Graphql::Authorize::AuthorizeResource concern in the mutation.

This allows us to provide the required abilities on the mutation like this:

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! and authorized_find! helper methods.

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.

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 params, 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:

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

it 'returns a successfull 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