How To: Create a new GraphQL mutation

Step 1: Identify which plugin owns the mutation

The complete Reaction Commerce GraphQL API is created by stitching together domain-specific APIs from all of the API plugins. So when adding a new mutation, the first step is to decide which plugin should own it. This is usually obvious, but not always. You should think about whether any other plugins or services will need to call your mutation. If the mutation is fundamental to the system, then it may need to go in the "core" plugin, if no better alternative exists.

Step 2: Understand the difference between a plugin mutation function and a GraphQL mutation resolver

See Resolver Mutations and Queries vs. Plugin Mutations and Queries

Step 3: Define the mutation in the schema

If it doesn't already exist, create schemas folder in the plugin, and add an index.js file there.
If it doesn't already exist, create schema.graphql in schemas in the plugin.

Import the GraphQL file into index.js and default export it in an array:

import importAsString from "@reactioncommerce/api-utils/importAsString.js";

const schema = importAsString("./schema.graphql");

export default [schema];

NOTE: For large plugins, you can split to multiple .graphql files and export a multi-item array.

In the .graphql file, add your mutation within extend type Mutation { }. Add an extend type Mutation section near the top if the file doesn't have it yet.
Follow the Relay recommendations for mutation input arguments, which is to have only one argument named input that takes an input type that is the capitalized mutation name plus the suffix "Input", and to return a type that is the capitalized mutation name plus the suffix "Payload".

Example: addAccountEmailRecord(input: AddAccountEmailRecordInput!): AddAccountEmailRecordPayload
Add the Input and Payload types to the schema. Both must have clientMutationId: String field and may have any other fields as necessary. The mutation response payload should include whatever object was mutated.
Document your mutation, the new types, and all fields in those types using string literals. See Documenting a GraphQL Schema.

If not already done, register your schemas in the plugin's index.js file:

import schemas from "./schemas";

export default async function register(app) {
  await app.registerPlugin({
    graphQL: {
      schemas
    },
    // other props
  });
}

Step 4: Create the plugin mutation file

If it doesn't already exist, create mutations folder in the plugin, and add an index.js file there.
In mutations, create a file for the mutation, e.g. createSomething.js for the createSomething mutation. The file should look something like this:

import Logger from "@reactioncommerce/logger";

/**
 * @method createSomething
 * @summary TODO
 * @param {Object} context - an object containing the per-request state
 * @return {Promise<Object>} TODO
 */
export default async function createSomething(context) {
  Logger.info("createSomething mutation is not yet implemented");
  return null;
}

Step 5: Add the plugin mutation to the mutations context

In mutations/index.js in the plugin, import your mutation and add it to the default export object. Example:

import createSomething from "./createSomething"

export default {
  createSomething
};

If this is the first mutation for the plugin, you'll also need to pass the full mutations object to registerPlugin in the plugin's index.js file:

import mutations from "./mutations";

export default async function register(app) {
  await app.registerPlugin({
    mutations,
    // other props
  });
}

Your plugin mutation function is now available in the GraphQL context as context.mutations.createSomething.

NOTE: The mutations objects from all plugins are merged, so be sure that another plugin does not have a mutation with the same name. The last one registered with that name will win, and plugins are generally registered in alphabetical order by plugin name. Tip: You can use this to your advantage if you want to override the mutation function of a core plugin without modifying core code.

Step 6: Add a test file for your mutation

If your mutation is in a file named createSomething.js, your Jest tests should be in a file named createSomething.test.js in the same folder. Initially you can copy and paste the following test:

import mockContext from "/imports/test-utils/helpers/mockContext";
import createSomething from "./createSomething";

test("expect to return a Promise that resolves to null", async () => {
  const result = await createSomething(mockContext);
  expect(result).toEqual(null);
});

This of course should be updated with tests that are appropriate for whatever your mutation does.

Step 7: Create the GraphQL mutation resolver file

If it doesn't already exist, create resolvers folder in the plugin, and add an index.js file there.
If it doesn't already exist, create resolvers/Mutation folder in the plugin, and add an index.js file there. "Mutation" must be capitalized.
In resolvers/Mutation, create a file for the mutation resolver, e.g. createSomething.js for the createSomething mutation. The file should look something like this initially:

/**
 * @name "Mutation.createSomething"
 * @method
 * @memberof MyPlugin/GraphQL
 * @summary resolver for the createSomething GraphQL mutation
 * @param {Object} parentResult - unused
 * @param {Object} args.input - an object of all mutation arguments that were sent by the client
 * @param {String} [args.input.clientMutationId] - An optional string identifying the mutation call
 * @param {Object} context - an object containing the per-request state
 * @return {Promise<Object>} CreateSomethingPayload
 */
export default async function createSomething(parentResult, { input }, context) {
  const { clientMutationId = null } = input;
  // TODO: decode incoming IDs here
  const renameMe = await context.mutations.createSomething(context);
  return {
    renameMe,
    clientMutationId
  };
}

Make adjustments to the resolver function so that it reads and passes along the parameters correctly. The general pattern is:

Destructure input. Include clientMutationId.
Decode any opaque IDs that are in the input object
Call context.mutations.<mutationName> (your new plugin mutation) with the necessary arguments, and await a response.
Return an object that contains the clientMutationId and the object returned by the plugin mutation. (This must match the "Payload" type from the schema.)

Step 8: Register the resolver

In resolvers/Mutation/index.js in the plugin, import your mutation resolver and add it to the default export object. Example:

import createSomething from "./createSomething"

export default {
  createSomething
};

If this is the first mutation for the plugin, you'll also need to import the Mutation object into the resolvers object. In resolvers/index.js in the plugin, import Mutation and add it to the default export object.

import Mutation from "./Mutation"

export default {
  Mutation
};

Then pass the full resolvers object to registerPlugin in the plugin's index.js file:

import resolvers from "./resolvers";
import schemas from "./schemas";

export default async function register(app) {
  await app.registerPlugin({
    graphQL: {
      resolvers,
      schemas
    },
    // other props
  });
}

Calling your mutation with GraphQL should now work.

Step 9: Add a test file for your mutation resolver

If your mutation resolver is in a file named createSomething.js, your Jest tests should be in a file named createSomething.test.js in the same folder. Initially you can copy and paste the following test:

import createSomething from "./createSomething";

test("correctly passes through to mutations.createSomething", async () => {
  const fakeResult = { /* TODO */ };

  const mockMutation = jest.fn().mockName("mutations.createSomething");
  mockMutation.mockReturnValueOnce(Promise.resolve(fakeResult));
  const context = {
    mutations: {
      createSomething: mockMutation
    }
  };

  const result = await createSomething(null, {
    input: {
      /* TODO */
      clientMutationId: "clientMutationId"
    }
  }, context);

  expect(result).toEqual({
    renameMe: fakeResult,
    clientMutationId: "clientMutationId"
  });
});

This of course should be updated with tests that are appropriate for whatever your mutation resolver does. For example, verify that all ID and schema transformations happen.

Step 10: Finish implementing your mutation

Adjust the mutation function and the mutation resolver function until they work as expected, with tests that prove it. This will likely involve adding additional input fields, ID transformations, permission checks, MongoDB calls, and event emitting.

Refer to Developing the GraphQL API for answers to any questions you might have while implementing your mutation.

Step 11: Update the JSDoc comments

Write/update jsdoc comments for the plugin mutation function, the mutation resolver, and any util functions. The resolver function must have @memberof <PluginName>/GraphQL in the jsdoc, and the @name must be the full GraphQL schema path in quotation marks, e.g., "Mutation.createSomething". (The quotation marks are necessary for the output API documentation to be correct due to the periods.)

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