Add mutation concern to resolve single issuable

- Add Mutations::ResolvesIssuable to resolve a
single issue or merge request using existing
ResolvesProject
- Add concern specs
- Replace existing code with same functionality
- Rename resolver method in ResolvesGroup
and ResolvesProject to specify resolver class

app/graphql/mutations/concerns/mutations/resolves_group.rb

@@ -5,10 +5,10 @@ module Mutations
     extend ActiveSupport::Concern
 
     def resolve_group(full_path:)
-      resolver.resolve(full_path: full_path)
+      group_resolver.resolve(full_path: full_path)
     end
 
-    def resolver
+    def group_resolver
       Resolvers::GroupResolver.new(object: nil, context: context)
     end
   end

app/graphql/mutations/concerns/mutations/resolves_issuable.rb

+# frozen_string_literal: true
+
+module Mutations
+  module ResolvesIssuable
+    extend ActiveSupport::Concern
+    include Mutations::ResolvesProject
+
+    def resolve_issuable(type:, parent_path:, iid:)
+      parent = resolve_issuable_parent(parent_path)
+
+      issuable_resolver(type, parent, context).resolve(iid: iid.to_s)
+    end
+
+    def issuable_resolver(type, parent, context)
+      resolver_class = "Resolvers::#{type.to_s.classify.pluralize}Resolver".constantize
+
+      resolver_class.single.new(object: parent, context: context)
+    end
+
+    def resolve_issuable_parent(parent_path)
+      resolve_project(full_path: parent_path)
+    end
+  end
+end

app/graphql/mutations/concerns/mutations/resolves_project.rb

@@ -5,10 +5,10 @@ module Mutations
     extend ActiveSupport::Concern
 
     def resolve_project(full_path:)
-      resolver.resolve(full_path: full_path)
+      project_resolver.resolve(full_path: full_path)
     end
 
-    def resolver
+    def project_resolver
       Resolvers::ProjectResolver.new(object: nil, context: context)
     end
   end

app/graphql/mutations/issues/base.rb

@@ -3,7 +3,7 @@
 module Mutations
   module Issues
     class Base < BaseMutation
-      include Mutations::ResolvesProject
+      include Mutations::ResolvesIssuable
 
       argument :project_path, GraphQL::ID_TYPE,
                required: true,
@@ -23,11 +23,7 @@ module Mutations
       private
 
       def find_object(project_path:, iid:)
-        project = resolve_project(full_path: project_path)
-        resolver = Resolvers::IssuesResolver
-          .single.new(object: project, context: context)
-
-        resolver.resolve(iid: iid)
+        resolve_issuable(type: :issue, parent_path: project_path, iid: iid)
       end
     end
   end

app/graphql/mutations/merge_requests/base.rb

@@ -3,7 +3,7 @@
 module Mutations
   module MergeRequests
     class Base < BaseMutation
-      include Mutations::ResolvesProject
+      include Mutations::ResolvesIssuable
 
       argument :project_path, GraphQL::ID_TYPE,
                required: true,
@@ -23,11 +23,7 @@ module Mutations
       private
 
       def find_object(project_path:, iid:)
-        project = resolve_project(full_path: project_path)
-        resolver = Resolvers::MergeRequestsResolver
-          .single.new(object: project, context: context)
-
-        resolver.resolve(iid: iid)
+        resolve_issuable(type: :merge_request, parent_path: project_path, iid: iid)
       end
     end
   end

ee/app/graphql/mutations/design_management/base.rb

@@ -3,7 +3,7 @@
 module Mutations
   module DesignManagement
     class Base < ::Mutations::BaseMutation
-      include Mutations::ResolvesProject
+      include Mutations::ResolvesIssuable
 
       argument :project_path, GraphQL::ID_TYPE,
                required: true,
@@ -16,10 +16,7 @@ module Mutations
       private
 
       def find_object(project_path:, iid:)
-        project = resolve_project(full_path: project_path)
-
-        Resolvers::IssuesResolver.single.new(object: project, context: context)
-          .resolve(iid: iid)
+        resolve_issuable(type: :issue, parent_path: project_path, iid: iid)
       end
     end
   end

spec/graphql/mutations/concerns/mutations/resolves_issuable_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Mutations::ResolvesIssuable do
+  let(:mutation_class) do
+    Class.new(Mutations::BaseMutation) do
+      include Mutations::ResolvesIssuable
+    end
+  end
+
+  let(:project)  { create(:project) }
+  let(:user)     { create(:user) }
+  let(:context)  { { current_user: user } }
+  let(:mutation) { mutation_class.new(object: nil, context: context) }
+
+  shared_examples 'resolving an issuable' do |type|
+    context 'when user has access' do
+      let(:source) { type == :merge_request ? 'source_project' : 'project' }
+      let(:issuable) { create(type, author: user, "#{source}" => project) }
+
+      subject { mutation.resolve_issuable(type: type, parent_path: project.full_path, iid: issuable.iid) }
+
+      before do
+        project.add_developer(user)
+      end
+
+      it 'resolves issuable by iid' do
+        result = type == :merge_request ? subject.sync : subject
+        expect(result).to eq(issuable)
+      end
+
+      it 'uses the correct Resolver to resolve issuable' do
+        resolver_class = "Resolvers::#{type.to_s.classify.pluralize}Resolver".constantize
+        resolved_project = mutation.resolve_project(full_path: project.full_path)
+
+        allow(mutation).to receive(:resolve_project)
+          .with(full_path: project.full_path)
+          .and_return(resolved_project)
+
+        expect(resolver_class).to receive(:new)
+          .with(object: resolved_project, context: context)
+          .and_call_original
+
+        subject
+      end
+
+      it 'uses the ResolvesProject to resolve project' do
+        expect(Resolvers::ProjectResolver).to receive(:new)
+          .with(object: nil, context: context)
+          .and_call_original
+
+        subject
+      end
+
+      it 'returns nil if issuable is not found' do
+        result = mutation.resolve_issuable(type: type, parent_path: project.full_path, iid: "100")
+        result = type == :merge_request ? result.sync : result
+
+        expect(result).to be_nil
+      end
+    end
+  end
+
+  context 'with issues' do
+    it_behaves_like 'resolving an issuable', :issue
+  end
+
+  context 'with merge requests' do
+    it_behaves_like 'resolving an issuable', :merge_request
+  end
+end