Use inheritance for stubbed class

- Add specs for setting parent in issues finder

spec/finders/issues_finder_spec.rb

@@ -3,6 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe IssuesFinder do
+  using RSpec::Parameterized::TableSyntax
   include_context 'IssuesFinder context'
 
   describe '#execute' do
@@ -1022,4 +1023,33 @@ RSpec.describe IssuesFinder do
       end
     end
   end
+
+  describe '#parent_param=' do
+    let(:finder) { described_class.new(nil) }
+
+    subject { finder.parent_param = obj }
+
+    where(:klass, :param) do
+      :Project | :project_id
+      :Group   | :group_id
+    end
+
+    with_them do
+      let(:obj) { Object.const_get(klass, false).new }
+
+      it 'sets the params' do
+        subject
+
+        expect(finder.params[param]).to eq(obj)
+      end
+    end
+
+    context 'unexpected parent' do
+      let(:obj) { MergeRequest.new }
+
+      it 'raises an error' do
+        expect { subject }.to raise_error('Unexpected parent: MergeRequest')
+      end
+    end
+  end
 end

spec/lib/gitlab/graphql/loaders/issuable_loader_spec.rb

@@ -11,33 +11,15 @@ RSpec.describe Gitlab::Graphql::Loaders::IssuableLoader do
   # Dumb finder class, that only implements what we need, and has
   # predictable query counts.
   let(:finder_class) do
-    Class.new do
-      attr_reader :current_user, :params
-      attr_accessor :parent
-
-      def initialize(user, args)
-        @current_user = user
-        @params = HashWithIndifferentAccess.new(args.to_h)
-      end
-
+    Class.new(IssuesFinder) do
       def execute
         params[:project_id].issues.where(iid: params[:iids])
       end
 
-      def parent_param=(obj)
-        @parent = obj
-        params[parent_param] = parent if parent
-      end
+      private
 
-      def parent_param
-        case parent
-        when Project
-          :project_id
-        when Group
-          :group_id
-        else
-          raise "Unexpected parent: #{parent.class}"
-        end
+      def params_class
+        IssuesFinder::Params
       end
     end
   end