Allow indexing_only relationships to reference non-indexed source types

Author: ellisandrews-toast

elasticgraph-schema_definition/lib/elastic_graph/schema_definition/indexing/relationship_resolver.rb

@@ -36,8 +36,12 @@ def resolve
               end
 
             [nil, "#{relationship_error_prefix} #{issue}"]
-          elsif !related_type.root_document_type?
-            [nil, "#{relationship_error_prefix} references a type which is not a root document type: `#{related_type.name}`. Only root document types can be used in relations."]
+          elsif !related_type.root_document_type? && !relationship.indexing_only
+            [nil, "#{relationship_error_prefix} references a type which is not a root document type: `#{related_type.name}`. " \
+              "Only root document types can be used in relations exposed to GraphQL. To relate to a non-indexed source type " \
+              "purely for indexing purposes (e.g. `sourced_from`), declare the relationship with `indexing_only: true`."]
+          elsif (id_field_error = validate_non_indexed_related_type_id_field(related_type))
+            [nil, id_field_error]
           else
             relation_metadata = relationship.runtime_metadata # : SchemaArtifacts::RuntimeMetadata::Relation
             foreign_key_parent_type = (relation_metadata.direction == :in) ? related_type : object_type
@@ -67,6 +71,18 @@ def relationship_error_prefix
           "`#{relationship_description}`#{sourced_fields_description}"
         end
 
+        # A non-indexed related type never goes through the `id` validation that `t.index` performs on indexed
+        # types, but relationships join on `id`, so we must verify it here.
+        def validate_non_indexed_related_type_id_field(related_type)
+          return nil if related_type.root_document_type?
+
+          id_field = schema_def_state.field_path_resolver.resolve_public_path(related_type, "id") { true }
+          return nil if id_field
+
+          "#{relationship_error_prefix} references `#{related_type.name}`, which lacks an `id` field. Related types " \
+            "must define an `id` field since relationships join on it."
+        end
+
         def validate_foreign_key(foreign_key_parent_type, relation_metadata)
           foreign_key_field = schema_def_state.field_path_resolver.resolve_public_path(foreign_key_parent_type, relation_metadata.foreign_key) { true }
           # If it's an inbound foreign key, verify that the foreign key exists on the related type.

elasticgraph-schema_definition/sig/elastic_graph/schema_definition/indexing/relationship_resolver.rbs

@@ -19,6 +19,7 @@ module ElasticGraph
         attr_reader sourced_fields: ::Array[SchemaElements::Field]
 
         def relationship_error_prefix: () -> ::String
+        def validate_non_indexed_related_type_id_field: (indexableType) -> ::String?
         def validate_foreign_key: (
           indexableType,
           SchemaArtifacts::RuntimeMetadata::Relation

elasticgraph-schema_definition/spec/unit/elastic_graph/schema_definition/runtime_metadata/object_types_by_name/update_targets_spec.rb

@@ -732,7 +732,7 @@ def expect_widget_update_target_with(
               )
             end
 
-            it "raises an error if the related type exists but is a non-indexed object type, regardless of whether there are any `sourced_from` fields or not" do
+            it "raises an error if the related type exists but is a non-indexed object type on a GraphQL-exposed relationship, regardless of whether there are any `sourced_from` fields or not" do
               expect {
                 update_targets_for("Widget", index_widget_workspaces: false) do |t|
                   t.relates_to_one "workspace", "WidgetWorkspace", via: "widget_ids", dir: :in
@@ -746,7 +746,7 @@ def expect_widget_update_target_with(
                   end
                 end
               }.to raise_error_about_workspace_relationship(
-                "references a type which is not a root document type: `WidgetWorkspace`. Only root document types can be used in relations."
+                "references a type which is not a root document type: `WidgetWorkspace`. Only root document types can be used in relations exposed to GraphQL."
               )
 
               expect {
@@ -755,11 +755,49 @@ def expect_widget_update_target_with(
                   expect(t.fields_with_sources).to be_empty
                 end
               }.to raise_error_about_workspace_relationship(
-                "references a type which is not a root document type: `WidgetWorkspace`. Only root document types can be used in relations.",
+                "references a type which is not a root document type: `WidgetWorkspace`. Only root document types can be used in relations exposed to GraphQL.",
                 sourced_fields: false
               )
             end
 
+            it "allows an `indexing_only: true` relationship to reference a non-indexed object type, so `sourced_from` source types need not be indexed" do
+              update_targets = update_targets_for("WidgetWorkspace", index_widget_workspaces: false) do |t|
+                t.relates_to_one "workspace", "WidgetWorkspace", via: "widget_ids", dir: :in, indexing_only: true
+
+                t.field "workspace_name", "String" do |f|
+                  f.sourced_from "workspace", "name"
+                end
+              end
+
+              expect_widget_update_target_with(
+                update_targets,
+                id_source: "widget_ids",
+                routing_value_source: nil,
+                relationship: "workspace",
+                top_level_fields_params: {
+                  "workspace_name" => dynamic_param_with(source_path: "name", cardinality: :one)
+                }
+              )
+            end
+
+            it "raises an error if a non-indexed related type lacks an `id` field, since relationships join on `id` and non-indexed types skip the `id` validation `t.index` performs" do
+              expect {
+                update_targets_for("Widget", index_widget_workspaces: false, define_widget_workspace_id: false) do |t|
+                  t.relates_to_one "workspace", "WidgetWorkspace", via: "widget_ids", dir: :in, indexing_only: true
+
+                  t.field "workspace_name", "String" do |f|
+                    f.sourced_from "workspace", "name"
+                  end
+
+                  t.field "workspace_created_at", "DateTime" do |f|
+                    f.sourced_from "workspace", "created_at"
+                  end
+                end
+              }.to raise_error_about_workspace_relationship(
+                "references `WidgetWorkspace`, which lacks an `id` field. Related types must define an `id` field since relationships join on it."
+              )
+            end
+
             it "raises an error if an inbound foreign key field does not exist on the related type, regardless of whether there are any `sourced_from` fields or not" do
               expect {
                 update_targets_for("Widget") do |t|
@@ -2039,6 +2077,7 @@ def update_targets_for(
           on_widgets_index: nil,
           on_widget_workspace_type: nil,
           index_widget_workspaces: true,
+          define_widget_workspace_id: true,
           type_name_overrides: {},
           &define_relation_and_sourced_from_fields
         )
@@ -2086,7 +2125,7 @@ def update_targets_for(
             end
 
             s.object_type "WidgetWorkspace" do |t|
-              t.field "id", "ID!"
+              t.field "id", "ID!" if define_widget_workspace_id
               t.field "workspace_owner_id", "ID!"
               t.field "name", widget_workspace_name, **widget_workspace_name_opts
               t.field "created_at", widget_workspace_created_at