Use ResolveResourceParent mixing for iterations

This dries a little bit the code by reusing same logic from mixin.

doc/api/graphql/reference/gitlab_schema.graphql

@@ -5095,12 +5095,12 @@ input CreateIterationInput {
   dueDate: String
 
   """
-  The target group for the iteration.
+  Full path of the group with which the resource is associated.
   """
   groupPath: ID
 
   """
-  The target project for the iteration.
+  Full path of the project with which the resource is associated.
   """
   projectPath: ID
 

doc/api/graphql/reference/gitlab_schema.json

@@ -13827,8 +13827,8 @@
           "fields": null,
           "inputFields": [
             {
-              "name": "groupPath",
-              "description": "The target group for the iteration.",
+              "name": "projectPath",
+              "description": "Full path of the project with which the resource is associated.",
               "type": {
                 "kind": "SCALAR",
                 "name": "ID",
@@ -13837,8 +13837,8 @@
               "defaultValue": null
             },
             {
-              "name": "projectPath",
-              "description": "The target project for the iteration.",
+              "name": "groupPath",
+              "description": "Full path of the group with which the resource is associated.",
               "type": {
                 "kind": "SCALAR",
                 "name": "ID",

ee/app/graphql/mutations/iterations/create.rb

@@ -3,8 +3,7 @@
 module Mutations
   module Iterations
     class Create < BaseMutation
-      include Mutations::ResolvesGroup
-      include ResolvesProject
+      include Mutations::ResolvesResourceParent
 
       graphql_name 'CreateIteration'
 
@@ -15,14 +14,6 @@ module Mutations
             null: true,
             description: 'The created iteration.'
 
-      argument :group_path, GraphQL::ID_TYPE,
-               required: false,
-               description: "The target group for the iteration."
-
-      argument :project_path, GraphQL::ID_TYPE,
-               required: false,
-               description: "The target project for the iteration."
-
       argument :title,
                GraphQL::STRING_TYPE,
                required: false,
@@ -46,7 +37,7 @@ module Mutations
       def resolve(args)
         validate_arguments!(args)
 
-        parent = find_parent(args)
+        parent = authorized_resource_parent_find!(args)
 
         response = ::Iterations::CreateService.new(parent, current_user, args).execute
 
@@ -61,40 +52,11 @@ module Mutations
 
       private
 
-      def find_object(group_path: nil, project_path: nil)
-        if group_path
-          resolve_group(full_path: group_path)
-        elsif project_path
-          resolve_project(full_path: project_path)
-        end
-      end
-
-      def find_parent(args)
-        group_path = args.delete(:group_path)
-        project_path = args.delete(:project_path)
-
-        if group_path
-          authorized_find!(group_path: group_path)
-        elsif project_path
-          authorized_find!(project_path: project_path)
-        end
-      end
-
       def validate_arguments!(args)
         if args.except(:group_path, :project_path).empty?
           raise Gitlab::Graphql::Errors::ArgumentError,
                 'The list of iteration attributes is empty'
         end
-
-        if args[:group_path].present? && args[:project_path].present?
-          raise Gitlab::Graphql::Errors::ArgumentError,
-                'Only one of group_path or project_path can be provided'
-        end
-
-        if args[:group_path].nil? && args[:project_path].nil?
-          raise Gitlab::Graphql::Errors::ArgumentError,
-                'Either group_path or project_path is required'
-        end
       end
     end
   end

ee/spec/requests/api/graphql/mutations/iterations/create_spec.rb

@@ -128,7 +128,7 @@ RSpec.describe 'Creating an Iteration' do
         let(:params) { {} }
 
         it_behaves_like 'a mutation that returns top-level errors',
-                        errors: ['Either group_path or project_path is required']
+                        errors: ['Exactly one of group_path or project_path arguments is required']
 
         it 'does not create the iteration' do
           expect { post_graphql_mutation(mutation, current_user: current_user) }.not_to change(Iteration, :count)
@@ -139,7 +139,7 @@ RSpec.describe 'Creating an Iteration' do
         let(:params) { { group_path: group.full_path, project_path: 'doesnotreallymatter' } }
 
         it_behaves_like 'a mutation that returns top-level errors',
-                        errors: ['Only one of group_path or project_path can be provided']
+                        errors: ['Exactly one of group_path or project_path arguments is required']
 
         it 'does not create the iteration' do
           expect { post_graphql_mutation(mutation, current_user: current_user) }.not_to change(Iteration, :count)