Add mutation to create a merge request in GraphQL

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/215619

* Generate documentation for a the GraphQL mutation

app/graphql/mutations/merge_requests/create.rb

+# frozen_string_literal: true
+
+module Mutations
+  module MergeRequests
+    class Create < BaseMutation
+      include Mutations::ResolvesProject
+
+      graphql_name 'MergeRequestCreate'
+
+      argument :project_path, GraphQL::ID_TYPE,
+               required: true,
+               description: 'Project full path the merge request is associated with'
+
+      argument :title, GraphQL::STRING_TYPE,
+               required: true,
+               description: copy_field_description(Types::MergeRequestType, :title)
+
+      argument :source_branch, GraphQL::STRING_TYPE,
+               required: true,
+               description: copy_field_description(Types::MergeRequestType, :source_branch)
+
+      argument :target_branch, GraphQL::STRING_TYPE,
+               required: true,
+               description: copy_field_description(Types::MergeRequestType, :target_branch)
+
+      argument :description, GraphQL::STRING_TYPE,
+               required: false,
+               description: copy_field_description(Types::MergeRequestType, :description)
+
+      field :merge_request,
+            Types::MergeRequestType,
+            null: true,
+            description: 'The merge request after mutation'
+
+      authorize :create_merge_request_from
+
+      def resolve(project_path:, title:, source_branch:, target_branch:, description: nil)
+        project = authorized_find!(full_path: project_path)
+
+        attributes = {
+          title: title,
+          source_branch: source_branch,
+          target_branch: target_branch,
+          author_id: current_user.id,
+          description: description
+        }
+
+        merge_request = ::MergeRequests::CreateService.new(project, current_user, attributes).execute
+
+        {
+          merge_request: merge_request.valid? ? merge_request : nil,
+          errors: errors_on_object(merge_request)
+        }
+      end
+
+      private
+
+      def find_object(full_path:)
+        resolve_project(full_path: full_path)
+      end
+    end
+  end
+end

app/graphql/types/mutation_type.rb

@@ -16,6 +16,7 @@ module Types
     mount_mutation Mutations::Issues::SetConfidential
     mount_mutation Mutations::Issues::SetDueDate
     mount_mutation Mutations::Issues::Update
+    mount_mutation Mutations::MergeRequests::Create
     mount_mutation Mutations::MergeRequests::SetLabels
     mount_mutation Mutations::MergeRequests::SetLocked
     mount_mutation Mutations::MergeRequests::SetMilestone

changelogs/unreleased/215619_add_mutation_to_create_mr.yml

+---
+title: Add mutation to create a merge request in GraphQL
+merge_request: 31867
+author:
+type: added

doc/api/graphql/reference/gitlab_schema.graphql

@@ -5976,6 +5976,61 @@ type MergeRequestConnection {
   pageInfo: PageInfo!
 }
 
+"""
+Autogenerated input type of MergeRequestCreate
+"""
+input MergeRequestCreateInput {
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+
+  """
+  Description of the merge request (Markdown rendered as HTML for caching)
+  """
+  description: String
+
+  """
+  Project full path the merge request is associated with
+  """
+  projectPath: ID!
+
+  """
+  Source branch of the merge request
+  """
+  sourceBranch: String!
+
+  """
+  Target branch of the merge request
+  """
+  targetBranch: String!
+
+  """
+  Title of the merge request
+  """
+  title: String!
+}
+
+"""
+Autogenerated return type of MergeRequestCreate
+"""
+type MergeRequestCreatePayload {
+  """
+  A unique identifier for the client performing the mutation.
+  """
+  clientMutationId: String
+
+  """
+  Errors encountered during execution of the mutation.
+  """
+  errors: [String!]!
+
+  """
+  The merge request after mutation
+  """
+  mergeRequest: MergeRequest
+}
+
 """
 An edge in a connection.
 """
@@ -6575,6 +6630,7 @@ type Mutation {
   issueSetWeight(input: IssueSetWeightInput!): IssueSetWeightPayload
   jiraImportStart(input: JiraImportStartInput!): JiraImportStartPayload
   markAsSpamSnippet(input: MarkAsSpamSnippetInput!): MarkAsSpamSnippetPayload
+  mergeRequestCreate(input: MergeRequestCreateInput!): MergeRequestCreatePayload
   mergeRequestSetAssignees(input: MergeRequestSetAssigneesInput!): MergeRequestSetAssigneesPayload
   mergeRequestSetLabels(input: MergeRequestSetLabelsInput!): MergeRequestSetLabelsPayload
   mergeRequestSetLocked(input: MergeRequestSetLockedInput!): MergeRequestSetLockedPayload

doc/api/graphql/reference/gitlab_schema.json

@@ -16689,6 +16689,160 @@
           "enumValues": null,
           "possibleTypes": null
         },
+        {
+          "kind": "INPUT_OBJECT",
+          "name": "MergeRequestCreateInput",
+          "description": "Autogenerated input type of MergeRequestCreate",
+          "fields": null,
+          "inputFields": [
+            {
+              "name": "projectPath",
+              "description": "Project full path the merge request is associated with",
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "ID",
+                  "ofType": null
+                }
+              },
+              "defaultValue": null
+            },
+            {
+              "name": "title",
+              "description": "Title of the merge request",
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "String",
+                  "ofType": null
+                }
+              },
+              "defaultValue": null
+            },
+            {
+              "name": "sourceBranch",
+              "description": "Source branch of the merge request",
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "String",
+                  "ofType": null
+                }
+              },
+              "defaultValue": null
+            },
+            {
+              "name": "targetBranch",
+              "description": "Target branch of the merge request",
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "SCALAR",
+                  "name": "String",
+                  "ofType": null
+                }
+              },
+              "defaultValue": null
+            },
+            {
+              "name": "description",
+              "description": "Description of the merge request (Markdown rendered as HTML for caching)",
+              "type": {
+                "kind": "SCALAR",
+                "name": "String",
+                "ofType": null
+              },
+              "defaultValue": null
+            },
+            {
+              "name": "clientMutationId",
+              "description": "A unique identifier for the client performing the mutation.",
+              "type": {
+                "kind": "SCALAR",
+                "name": "String",
+                "ofType": null
+              },
+              "defaultValue": null
+            }
+          ],
+          "interfaces": null,
+          "enumValues": null,
+          "possibleTypes": null
+        },
+        {
+          "kind": "OBJECT",
+          "name": "MergeRequestCreatePayload",
+          "description": "Autogenerated return type of MergeRequestCreate",
+          "fields": [
+            {
+              "name": "clientMutationId",
+              "description": "A unique identifier for the client performing the mutation.",
+              "args": [
+
+              ],
+              "type": {
+                "kind": "SCALAR",
+                "name": "String",
+                "ofType": null
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            },
+            {
+              "name": "errors",
+              "description": "Errors encountered during execution of the mutation.",
+              "args": [
+
+              ],
+              "type": {
+                "kind": "NON_NULL",
+                "name": null,
+                "ofType": {
+                  "kind": "LIST",
+                  "name": null,
+                  "ofType": {
+                    "kind": "NON_NULL",
+                    "name": null,
+                    "ofType": {
+                      "kind": "SCALAR",
+                      "name": "String",
+                      "ofType": null
+                    }
+                  }
+                }
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            },
+            {
+              "name": "mergeRequest",
+              "description": "The merge request after mutation",
+              "args": [
+
+              ],
+              "type": {
+                "kind": "OBJECT",
+                "name": "MergeRequest",
+                "ofType": null
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            }
+          ],
+          "inputFields": null,
+          "interfaces": [
+
+          ],
+          "enumValues": null,
+          "possibleTypes": null
+        },
         {
           "kind": "OBJECT",
           "name": "MergeRequestEdge",
@@ -19145,6 +19299,33 @@
               "isDeprecated": false,
               "deprecationReason": null
             },
+            {
+              "name": "mergeRequestCreate",
+              "description": null,
+              "args": [
+                {
+                  "name": "input",
+                  "description": null,
+                  "type": {
+                    "kind": "NON_NULL",
+                    "name": null,
+                    "ofType": {
+                      "kind": "INPUT_OBJECT",
+                      "name": "MergeRequestCreateInput",
+                      "ofType": null
+                    }
+                  },
+                  "defaultValue": null
+                }
+              ],
+              "type": {
+                "kind": "OBJECT",
+                "name": "MergeRequestCreatePayload",
+                "ofType": null
+              },
+              "isDeprecated": false,
+              "deprecationReason": null
+            },
             {
               "name": "mergeRequestSetAssignees",
               "description": null,

doc/api/graphql/reference/index.md

@@ -874,6 +874,16 @@ Autogenerated return type of MarkAsSpamSnippet
 | `webUrl` | String | Web URL of the merge request |
 | `workInProgress` | Boolean! | Indicates if the merge request is a work in progress (WIP) |
 
+## MergeRequestCreatePayload
+
+Autogenerated return type of MergeRequestCreate
+
+| Name  | Type  | Description |
+| ---   |  ---- | ----------  |
+| `clientMutationId` | String | A unique identifier for the client performing the mutation. |
+| `errors` | String! => Array | Errors encountered during execution of the mutation. |
+| `mergeRequest` | MergeRequest | The merge request after mutation |
+
 ## MergeRequestPermissions
 
 Check permissions for the current user on a merge request

spec/graphql/mutations/merge_requests/create_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Mutations::MergeRequests::Create do
+  subject(:mutation) { described_class.new(object: nil, context: context, field: nil) }
+
+  let_it_be(:project) { create(:project, :public, :repository) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:context) do
+    GraphQL::Query::Context.new(
+      query: OpenStruct.new(schema: nil),
+      values: { current_user: user },
+      object: nil
+    )
+  end
+
+  describe '#resolve' do
+    subject do
+      mutation.resolve(
+        project_path: project.full_path,
+        title: title,
+        source_branch: source_branch,
+        target_branch: target_branch,
+        description: description
+      )
+    end
+
+    let(:title) { 'MergeRequest' }
+    let(:source_branch) { 'feature' }
+    let(:target_branch) { 'master' }
+    let(:description) { nil }
+
+    let(:mutated_merge_request) { subject[:merge_request] }
+
+    it 'raises an error if the resource is not accessible to the user' do
+      expect { subject }.to raise_error(Gitlab::Graphql::Errors::ResourceNotAvailable)
+    end
+
+    context 'when user does not have enough permissions to create a merge request' do
+      before do
+        project.add_guest(user)
+      end
+
+      it 'raises an error if the resource is not accessible to the user' do
+        expect { subject }.to raise_error(Gitlab::Graphql::Errors::ResourceNotAvailable)
+      end
+    end
+
+    context 'when the user can create a merge request' do
+      before_all do
+        project.add_developer(user)
+      end
+
+      it 'creates a new merge request' do
+        expect { mutated_merge_request }.to change(MergeRequest, :count).by(1)
+      end
+
+      it 'returns a new merge request' do
+        expect(mutated_merge_request.title).to eq(title)
+        expect(subject[:errors]).to be_empty
+      end
+
+      context 'when optional description field is set' do
+        let(:description) { 'content' }
+
+        it 'returns a new merge request with a description' do
+          expect(mutated_merge_request.description).to eq(description)
+          expect(subject[:errors]).to be_empty
+        end
+      end
+
+      context 'when service cannot create a merge request' do
+        let(:title) { nil }
+
+        it 'does not create a new merge request' do
+          expect { mutated_merge_request }.not_to change(MergeRequest, :count)
+        end
+
+        it 'returns errors' do
+          expect(mutated_merge_request).to be_nil
+          expect(subject[:errors]).to eq(['Title can\'t be blank'])
+        end
+      end
+    end
+  end
+end

spec/requests/api/graphql/mutations/merge_requests/create_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Creation of a new merge request' do
+  include GraphqlHelpers
+
+  let_it_be(:current_user) { create(:user) }
+  let(:project) { create(:project, :public, :repository) }
+  let(:input) do
+    {
+      project_path: project.full_path,
+      title: title,
+      source_branch: source_branch,
+      target_branch: target_branch
+    }
+  end
+  let(:title) { 'MergeRequest' }
+  let(:source_branch) { 'new_branch' }
+  let(:target_branch) { 'master' }
+
+  let(:mutation) { graphql_mutation(:merge_request_create, input) }
+  let(:mutation_response) { graphql_mutation_response(:merge_request_create) }
+
+  context 'the user is not allowed to create a branch' do
+    it_behaves_like 'a mutation that returns top-level errors',
+      errors: ['The resource that you are attempting to access does not exist or you don\'t have permission to perform this action']
+  end
+
+  context 'when user has permissions to create a merge request' do
+    before do
+      project.add_developer(current_user)
+    end
+
+    it 'creates a new merge request' do
+      post_graphql_mutation(mutation, current_user: current_user)
+
+      expect(response).to have_gitlab_http_status(:success)
+      expect(mutation_response['mergeRequest']).to include(
+        'title' => title
+      )
+    end
+
+    context 'when source branch is equal to the target branch' do
+      let(:source_branch) { target_branch }
+
+      it_behaves_like 'a mutation that returns errors in the response',
+        errors: ['Branch conflict You can\'t use same project/branch for source and target']
+    end
+  end
+end