Enhance Global-ID handling

This splits out GID parsing, and adds support for asserting expected
types.

app/graphql/gitlab_schema.rb

@@ -58,19 +58,12 @@ class GitlabSchema < GraphQL::Schema
     end
 
     def object_from_id(global_id, ctx = {})
-      expected_type = ctx[:expected_type]
-      gid = GlobalID.parse(global_id)
+      gid = parse_gid(global_id, ctx)
 
-      unless gid
-        raise Gitlab::Graphql::Errors::ArgumentError, "#{global_id} is not a valid GitLab id."
-      end
-
-      if expected_type && !gid.model_class.ancestors.include?(expected_type)
-        vars = { global_id: global_id, expected_type: expected_type }
-        msg = _('%{global_id} is not a valid id for %{expected_type}.') % vars
-        raise Gitlab::Graphql::Errors::ArgumentError, msg
+      find_by_gid(gid)
     end
 
+    def find_by_gid(gid)
       if gid.model_class < ApplicationRecord
         Gitlab::Graphql::Loaders::BatchModelLoader.new(gid.model_class, gid.model_id).find
       elsif gid.model_class.respond_to?(:lazy_find)
@@ -80,6 +73,21 @@ class GitlabSchema < GraphQL::Schema
       end
     end
 
+    def parse_gid(global_id, ctx = {})
+      expected_type = ctx[:expected_type]
+      gid = GlobalID.parse(global_id)
+
+      raise Gitlab::Graphql::Errors::ArgumentError, "#{global_id} is not a valid GitLab id." unless gid
+
+      if expected_type && !gid.model_class.ancestors.include?(expected_type)
+        vars = { global_id: global_id, expected_type: expected_type }
+        msg = _('%{global_id} is not a valid id for %{expected_type}.') % vars
+        raise Gitlab::Graphql::Errors::ArgumentError, msg
+      end
+
+      gid
+    end
+
     private
 
     def max_query_complexity(ctx)

spec/graphql/gitlab_schema_spec.rb

@@ -124,14 +124,26 @@ describe GitlabSchema do
 
   describe '.object_from_id' do
     context 'for subclasses of `ApplicationRecord`' do
-      it 'returns the correct record' do
-        user = create(:user)
+      let_it_be(:user) { create(:user) }
 
+      it 'returns the correct record' do
         result = described_class.object_from_id(user.to_global_id.to_s)
 
         expect(result.sync).to eq(user)
       end
 
+      it 'returns the correct record, of the expected type' do
+        result = described_class.object_from_id(user.to_global_id.to_s, expected_type: ::User)
+
+        expect(result.sync).to eq(user)
+      end
+
+      it 'fails if the type does not match' do
+        expect do
+          described_class.object_from_id(user.to_global_id.to_s, expected_type: ::Project)
+        end.to raise_error(Gitlab::Graphql::Errors::ArgumentError)
+      end
+
       it 'batchloads the queries' do
         user1 = create(:user)
         user2 = create(:user)