Improve detection of similarity sorting in GraphQL

This change annotates the Arel ORDER expression in SimilarityScore
for easier detection when it's used in GraphQL.

lib/gitlab/database/similarity_score.rb

@@ -6,6 +6,11 @@ module Gitlab
       EMPTY_STRING = Arel.sql("''").freeze
       EXPRESSION_ON_INVALID_INPUT = Arel::Nodes::NamedFunction.new('CAST', [Arel.sql('0').as('integer')]).freeze
       DEFAULT_MULTIPLIER = 1
+      DISPLAY_NAME = self.name.underscore.freeze
+
+      # Adds a "magic" comment in the generated SQL expression in order to be able to tell if we're sorting by similarity.
+      # Example: /* gitlab/database/similarity_score */ SIMILARITY(COALESCE...
+      SIMILARITY_FUNCTION_CALL_WITH_ANNOTATION = "/* #{DISPLAY_NAME} */ SIMILARITY".freeze
 
       # This method returns an Arel expression that can be used in an ActiveRecord query to order the resultset by similarity.
       #
@@ -74,6 +79,10 @@ module Gitlab
         end
       end
 
+      def self.order_by_similarity?(arel_query)
+        arel_query.to_sql.include?(SIMILARITY_FUNCTION_CALL_WITH_ANNOTATION)
+      end
+
       # (SIMILARITY(COALESCE(column, ''), 'search_string') * CAST(multiplier AS numeric))
       def self.rule_to_arel(search, rule)
         Arel::Nodes::Grouping.new(
@@ -91,7 +100,7 @@ module Gitlab
 
       # SIMILARITY(COALESCE(column, ''), 'search_string')
       def self.similarity_function_call(search, column)
-        Arel::Nodes::NamedFunction.new('SIMILARITY', [column, Arel.sql(search)])
+        Arel::Nodes::NamedFunction.new(SIMILARITY_FUNCTION_CALL_WITH_ANNOTATION, [column, Arel.sql(search)])
       end
 
       # CAST(multiplier AS numeric)

lib/gitlab/graphql/pagination/keyset/order_info.rb

@@ -106,7 +106,7 @@ module Gitlab
 
           # determine if ordering using SIMILARITY scoring based on Gitlab::Database::SimilarityScore
           def ordering_by_similarity?(order_value)
-            order_value.to_sql.match?(/SIMILARITY\(.+\*/)
+            Gitlab::Database::SimilarityScore.order_by_similarity?(order_value)
           end
         end
       end

spec/lib/gitlab/database/similarity_score_spec.rb

@@ -90,4 +90,15 @@ RSpec.describe Gitlab::Database::SimilarityScore do
       expect(subject).to eq(%w[different same gitlab-danger])
     end
   end
+
+  describe 'annotation' do
+    it 'annotates the generated SQL expression' do
+      expression = Gitlab::Database::SimilarityScore.build_expression(search: 'test', rules: [
+        { column: Arel.sql('path'), multiplier: 1 },
+        { column: Arel.sql('name'), multiplier: 0.8 }
+      ])
+
+      expect(Gitlab::Database::SimilarityScore).to be_order_by_similarity(expression)
+    end
+  end
 end

spec/lib/gitlab/graphql/pagination/keyset/query_builder_spec.rb

@@ -136,11 +136,12 @@ RSpec.describe Gitlab::Graphql::Pagination::Keyset::QueryBuilder do
       let(:relation) { Project.sorted_by_similarity_desc('test', include_in_select: true) }
       let(:arel_table) { Project.arel_table }
       let(:decoded_cursor) { { 'similarity' => 0.5, 'id' => 100 } }
+      let(:similarity_function_call) { Gitlab::Database::SimilarityScore::SIMILARITY_FUNCTION_CALL_WITH_ANNOTATION }
       let(:similarity_sql) do
         [
-          '(SIMILARITY(COALESCE("projects"."path", \'\'), \'test\') * CAST(\'1\' AS numeric))',
-          '(SIMILARITY(COALESCE("projects"."name", \'\'), \'test\') * CAST(\'0.7\' AS numeric))',
-          '(SIMILARITY(COALESCE("projects"."description", \'\'), \'test\') * CAST(\'0.2\' AS numeric))'
+          "(#{similarity_function_call}(COALESCE(\"projects\".\"path\", ''), 'test') * CAST('1' AS numeric))",
+          "(#{similarity_function_call}(COALESCE(\"projects\".\"name\", ''), 'test') * CAST('0.7' AS numeric))",
+          "(#{similarity_function_call}(COALESCE(\"projects\".\"description\", ''), 'test') * CAST('0.2' AS numeric))"
         ].join(' + ')
       end