Rerverts query changes that did not return relation

Relates to https://gitlab.com/gitlab-org/gitlab/issues/293713

ee/app/models/ee/snippet_repository.rb

@@ -17,33 +17,30 @@ module EE
       def replicables_for_current_secondary(primary_key_in)
         node = ::Gitlab::Geo.current_node
 
-        if !node.selective_sync?
-          all.primary_key_in(primary_key_in)
-        elsif node.selective_sync_by_namespaces?
-          snippet_repositories_for_selected_namespaces(primary_key_in)
-        elsif node.selective_sync_by_shards?
-          snippet_repositories_for_selected_shards(primary_key_in)
-        else
-          self.none
-        end
+        replicables = if !node.selective_sync?
+                        all
+                      elsif node.selective_sync_by_namespaces?
+                        snippet_repositories_for_selected_namespaces
+                      elsif node.selective_sync_by_shards?
+                        snippet_repositories_for_selected_shards
+                      else
+                        self.none
+                      end
+
+        replicables.primary_key_in(primary_key_in)
       end
 
-      def snippet_repositories_for_selected_namespaces(primary_key_in)
-        personal_snippets = self.where(snippet: ::Snippet.only_personal_snippets.primary_key_in(primary_key_in))
+      def snippet_repositories_for_selected_namespaces
+        personal_snippets = self.joins(:snippet).where(snippet: ::Snippet.only_personal_snippets)
 
-        project_snippets = self.where(snippet: ::Snippet.for_projects(::Gitlab::Geo.current_node.projects.select(:id))
-                                                   .primary_key_in(primary_key_in))
+        project_snippets = self.joins(snippet: :project)
+                               .merge(::Snippet.for_projects(::Gitlab::Geo.current_node.projects.select(:id)))
 
-        # We use `find_by_sql` here in order to perform the union operation and cast the results as
-        # Snippet repositories. If we use a `from_union`, it wraps the query in in a sub-select and
-        # it increases the query time by a surprising amount.
-        self.find_by_sql(::Gitlab::SQL::Union.new([project_snippets, personal_snippets]).to_sql)
+        self.from_union([project_snippets, personal_snippets])
       end
 
-      def snippet_repositories_for_selected_shards(primary_key_in)
-        self
-          .for_repository_storage(::Gitlab::Geo.current_node.selective_sync_shards)
-          .primary_key_in(primary_key_in)
+      def snippet_repositories_for_selected_shards
+        self.for_repository_storage(::Gitlab::Geo.current_node.selective_sync_shards)
       end
     end