Enhance Global-ID handling

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

app/graphql/gitlab_schema.rb

@@ -58,12 +58,26 @@ class GitlabSchema < GraphQL::Schema
     end
 
     def object_from_id(global_id, ctx = {})
+      gid = parse_gid(global_id, ctx)
+
+      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)
+        gid.model_class.lazy_find(gid.model_id)
+      else
+        gid.find
+      end
+    end
+
+    def parse_gid(global_id, ctx = {})
       expected_type = ctx[:expected_type]
       gid = GlobalID.parse(global_id)
 
-      unless gid
-        raise Gitlab::Graphql::Errors::ArgumentError, "#{global_id} is not a valid GitLab id."
-      end
+      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 }
@@ -71,13 +85,7 @@ class GitlabSchema < GraphQL::Schema
         raise Gitlab::Graphql::Errors::ArgumentError, msg
       end
 
-      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)
-        gid.model_class.lazy_find(gid.model_id)
-      else
-        gid.find
-      end
+      gid
     end
 
     private

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)