gitlab-org--gitlab-foss/doc/development/testing_guide/contract/consumer_tests.md

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# Writing consumer tests

This tutorial guides you through writing a consumer test from scratch. To start, the consumer tests are written using [`jest-pact`](https://github.com/pact-foundation/jest-pact) that builds on top of [`pact-js`](https://github.com/pact-foundation/pact-js). This tutorial shows you how to write a consumer test for the `/discussions.json` endpoint, which is actually `/:namespace_name/:project_name/-/merge_requests/:id/discussions.json`.

## Create the skeleton

Start by creating the skeleton of a consumer test. Create a file under `spec/contracts/consumer/specs/project/merge_request` called `discussions.spec.js`.
Then, populate it with the following function and parameters:

- [`pactWith`](#the-pactwith-function)
- [`PactOptions`](#the-pactoptions-parameter)
- [`PactFn`](#the-pactfn-parameter)

To learn more about how the contract test directory is structured, see the contract testing [test suite folder structure](index.md#test-suite-folder-structure).

### The `pactWith` function

The Pact consumer test is defined through the `pactWith` function that takes `PactOptions` and the `PactFn`.

```javascript
const { pactWith } = require('jest-pact');

pactWith(PactOptions, PactFn);
```

### The `PactOptions` parameter

`PactOptions` with `jest-pact` introduces [additional options](https://github.com/pact-foundation/jest-pact/blob/dce370c1ab4b7cb5dff12c4b62246dc229c53d0e/README.md#defaults) that build on top of the ones [provided in `pact-js`](https://github.com/pact-foundation/pact-js#constructor). In most cases, you define the `consumer`, `provider`, `log`, and `dir` options for these tests.

```javascript
const { pactWith } = require('jest-pact');

pactWith(
  {
    consumer: 'MergeRequest#show',
    provider: 'Merge Request Discussions Endpoint',
    log: '../logs/consumer.log',
    dir: '../contracts/project/merge_request/show',
  },
  PactFn
);
```

To learn more about how to name the consumers and providers, see contract testing [naming conventions](index.md#naming-conventions).

### The `PactFn` parameter

The `PactFn` is where your tests are defined. This is where you set up the mock provider and where you can use the standard Jest methods like [`Jest.describe`](https://jestjs.io/docs/api#describename-fn), [`Jest.beforeEach`](https://jestjs.io/docs/api#beforeeachfn-timeout), and [`Jest.it`](https://jestjs.io/docs/api#testname-fn-timeout). For more information, see [https://jestjs.io/docs/api](https://jestjs.io/docs/api).

```javascript
const { pactWith } = require('jest-pact');

pactWith(
  {
    consumer: 'MergeRequest#show',
    provider: 'Merge Request Discussions Endpoint',
    log: '../logs/consumer.log',
    dir: '../contracts',
  },

  (provider) => {
    describe('Merge Request Discussions Endpoint', () => {
      beforeEach(() => {

      });

      it('return a successful body', () => {

      });
    });
  },
);
```

## Set up the mock provider

Before you run your test, set up the mock provider that handles the specified requests and returns a specified response. To do that, define the state and the expected request and response in an [`Interaction`](https://github.com/pact-foundation/pact-js/blob/master/src/dsl/interaction.ts).

For this tutorial, define four attributes for the `Interaction`:

1. `state`: A description of what the prerequisite state is before the request is made.
1. `uponReceiving`: A description of what kind of request this `Interaction` is handling.
1. `withRequest`: Where you define the request specifications. It contains the request `method`, `path`, and any `headers`, `body`, or `query`.
1. `willRespondWith`: Where you define the expected response. It contains the response `status`, `headers`, and `body`.

After you define the `Interaction`, add that interaction to the mock provider by calling `addInteraction`.

```javascript
const { pactWith } = require('jest-pact');
const { Matchers } = require('@pact-foundation/pact');

pactWith(
  {
    consumer: 'MergeRequest#show',
    provider: 'Merge Request Discussions Endpoint',
    log: '../logs/consumer.log',
    dir: '../contracts/project/merge_request/show',
  },

  (provider) => {
    describe('Merge Request Discussions Endpoint', () => {
      beforeEach(() => {
        const interaction = {
          state: 'a merge request with discussions exists',
          uponReceiving: 'a request for discussions',
          withRequest: {
            method: 'GET',
            path: '/gitlab-org/gitlab-qa/-/merge_requests/1/discussions.json',
            headers: {
              Accept: '*/*',
            },
          },
          willRespondWith: {
            status: 200,
            headers: {
              'Content-Type': 'application/json; charset=utf-8',
            },
            body: Matchers.eachLike({
              id: Matchers.string('fd73763cbcbf7b29eb8765d969a38f7d735e222a'),
              project_id: Matchers.integer(6954442),
              ...
              resolved: Matchers.boolean(true)
            }),
          },
        };
        provider.addInteraction(interaction);
      });

      it('return a successful body', () => {

      });
    });
  },
);
```

### Response body `Matchers`

Notice how we use `Matchers` in the `body` of the expected response. This allows us to be flexible enough to accept different values but still be strict enough to distinguish between valid and invalid values. We must ensure that we have a tight definition that is neither too strict nor too lax. Read more about the [different types of `Matchers`](https://github.com/pact-foundation/pact-js#using-the-v3-matching-rules).

## Write the test

After the mock provider is set up, you can write the test. For this test, you make a request and expect a particular response.

First, set up the client that makes the API request. To do that, create `spec/contracts/consumer/endpoints/project/merge_requests.js` and add the following API request.

```javascript
const axios = require('axios');

exports.getDiscussions = (endpoint) => {
  const url = endpoint.url;

  return axios
    .request({
      method: 'GET',
      baseURL: url,
      url: '/gitlab-org/gitlab-qa/-/merge_requests/1/discussions.json',
      headers: { Accept: '*/*' },
    })
    .then((response) => response.data);
};
```

After that's set up, import it to the test file and call it to make the request. Then, you can make the request and define your expectations.

```javascript
const { pactWith } = require('jest-pact');
const { Matchers } = require('@pact-foundation/pact');

const { getDiscussions } = require('../endpoints/project/merge_requests');

pactWith(
  {
    consumer: 'MergeRequest#show',
    provider: 'Merge Request Discussions Endpoint',
    log: '../logs/consumer.log',
    dir: '../contracts/project/merge_request/show',
  },

  (provider) => {
    describe('Merge Request Discussions Endpoint', () => {
      beforeEach(() => {
        const interaction = {
          state: 'a merge request with discussions exists',
          uponReceiving: 'a request for discussions',
          withRequest: {
            method: 'GET',
            path: '/gitlab-org/gitlab-qa/-/merge_requests/1/discussions.json',
            headers: {
              Accept: '*/*',
            },
          },
          willRespondWith: {
            status: 200,
            headers: {
              'Content-Type': 'application/json; charset=utf-8',
            },
            body: Matchers.eachLike({
              id: Matchers.string('fd73763cbcbf7b29eb8765d969a38f7d735e222a'),
              project_id: Matchers.integer(6954442),
              ...
              resolved: Matchers.boolean(true)
            }),
          },
        };
      });

      it('return a successful body', () => {
        return getDiscussions({
          url: provider.mockService.baseUrl,
        }).then((discussions) => {
          expect(discussions).toEqual(Matchers.eachLike({
            id: 'fd73763cbcbf7b29eb8765d969a38f7d735e222a',
            project_id: 6954442,
            ...
            resolved: true
          }));
        });
      });
    });
  },
);
```

There we have it! The consumer test is now set up. You can now try [running this test](index.md#run-the-consumer-tests).

## Improve test readability

As you may have noticed, the request and response definitions can get large. This results in the test being difficult to read, with a lot of scrolling to find what you want. You can make the test easier to read by extracting these out to a `fixture`.

Create a file under `spec/contracts/consumer/fixtures/project/merge_request` called `discussions.fixture.js` where you will place the `request` and `response` definitions.

```javascript
const { Matchers } = require('@pact-foundation/pact');

const body = Matchers.eachLike({
  id: Matchers.string('fd73763cbcbf7b29eb8765d969a38f7d735e222a'),
  project_id: Matchers.integer(6954442),
  ...
  resolved: Matchers.boolean(true)
});

const Discussions = {
  body: Matchers.extractPayload(body),

  success: {
    status: 200,
    headers: {
      'Content-Type': 'application/json; charset=utf-8',
    },
    body: body,
  },

  request: {
    uponReceiving: 'a request for discussions',
    withRequest: {
      method: 'GET',
      path: '/gitlab-org/gitlab-qa/-/merge_requests/1/discussions.json',
      headers: {
        Accept: '*/*',
      },
    },
  },
};

exports.Discussions = Discussions;
```

With all of that moved to the `fixture`, you can simplify the test to the following:

```javascript
const { pactWith } = require('jest-pact');

const { Discussions } = require('../fixtures/discussions.fixture');
const { getDiscussions } = require('../endpoints/project/merge_requests');

pactWith(
  {
    consumer: 'MergeRequest#show',
    provider: 'Merge Request Discussions Endpoint',
    log: '../logs/consumer.log',
    dir: '../contracts/project/merge_request/show',
  },

  (provider) => {
    describe('Merge Request Discussions Endpoint', () => {
      beforeEach(() => {
        const interaction = {
          state: 'a merge request with discussions exists',
          ...Discussions.request,
          willRespondWith: Discussions.success,
        };
        return provider.addInteraction(interaction);
      });

      it('return a successful body', () => {
        return getDiscussions({
          url: provider.mockService.baseUrl,
        }).then((discussions) => {
          expect(discussions).toEqual(Discussions.body);
        });
      });
    });
  },
);
```