fix: query planning for shared entities

[StarpTech]

Summary

Fixes query planning failure (500 error) when a federated query references a shared entity whose @key
field is marked @external in one subgraph.

Example: A CartBundleProduct entity exists in two subgraphs. The cart subgraph owns fields like name
and category but marks the key field code as @external. The products subgraph owns code. Querying
bundleProducts { code name } through the cart subgraph would fail with:

could not select the datasource to resolve CartBundleProduct.code

Root Cause

Newer federation composition puts @external key fields only in ExternalFieldNames (not in both
FieldNames and ExternalFieldNames like older composition did). This broke two assumptions in the
planner:

1. Key source detection (key_fields_visitor.go): The existing fallback that un-marks external key
   fields relied on the field being in both FieldNames and ExternalFieldNames. With the field only in
   ExternalFieldNames, the key was incorrectly treated as external, preventing the cart subgraph from
   being a key "source." This broke the jump graph so no datasource could resolve the field.
2. Field suggestion creation (datasource_filter_collect_nodes_visitor.go): Even after fixing #1, the
   cart subgraph still didn't suggest the code field. When the visitor converted an external field to
   non-external (via notExternalKeyPaths), it cleared isExternal but the field had no other qualifying
   flag (hasRootNode, hasChildNode, isProvided were all false). So no suggestion was created and the
   planner routed code to the products subgraph via entity resolution — generating a malformed upstream
   query.

Fix

Three changes:

• datasource_configuration.go: Add HasNonExternalFieldsForType() to check whether a datasource
  actively resolves a type (has regular, non-external fields for it) vs. being a stub with only external
  key fields.
• key_fields_visitor.go: When a key field is external but the datasource has non-external fields for
  that entity type, treat the key as non-external. The datasource actively resolves this entity, so it
  can provide the key field in its response.
• datasource_filter_collect_nodes_visitor.go: When an external field is reclassified as non-external
  (because it's a key field), promote its external root/child node status to regular root/child status.
  This ensures a field suggestion is actually created, so the planner can route the field to the correct
  datasource.

Test plan

• New test TestGraphQLDataSourceFederation_SharedEntityWithExternalKeyField with 4 sub-tests:
  • bundleProducts with code field — the main bug (was failing, now passes)
  • bundleProducts with only code field — variant of the bug (was failing, now passes)
  • cart only fields without bundleProducts — control (always passed)
  • bundleProducts without code field — control (always passed)

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CodeRabbit

Summary by CodeRabbit

Release Notes

• Tests

  • Added comprehensive test coverage for GraphQL federation with shared entities, including scenarios with external key fields and complex entity relationships.

• Improvements

  • Enhanced federation support for key fields defined in external field names across subgraphs.
  • Improved query planning and data source resolution in federated GraphQL schemas with shared entities.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.

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📥 Commits

Reviewing files that changed from the base of the PR and between 78ea8ff and e1da1c7.

📒 Files selected for processing (4)

• v2/pkg/engine/datasource/graphql_datasource/graphql_datasource_federation_shared_entity_test.go
• v2/pkg/engine/plan/datasource_configuration.go
• v2/pkg/engine/plan/datasource_filter_collect_nodes_visitor.go
• v2/pkg/engine/plan/key_fields_visitor.go

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📝 Walkthrough

Walkthrough

This PR introduces federation support for shared entities with external key fields. It adds a new test suite validating federation scenarios where key fields exist only in ExternalFieldNames rather than regular FieldNames, implements a new interface method to check for non-external fields in datasources, and updates key-field visitor logic to correctly treat keys as non-external when datasources actively resolve entities.

Changes

Cohort / File(s)	Summary
Federation Key-Field Handling v2/pkg/engine/plan/key_fields_visitor.go, v2/pkg/engine/plan/datasource_filter_collect_nodes_visitor.go	Added logic to treat keys as non-external when datasource reports non-external fields for the type; promotes node statuses when external fields are disabled during key-field handling to preserve query suggestions.
Federation Metadata Configuration v2/pkg/engine/plan/datasource_configuration.go	Introduced HasNonExternalFieldsForType interface method on NodesInfo and implementation on DataSourceMetadata to check existence of non-external fields for a given type.
Federation Shared Entity Tests v2/pkg/engine/datasource/graphql_datasource/graphql_datasource_federation_shared_entity_test.go	Added comprehensive test TestGraphQLDataSourceFederation_SharedEntityWithExternalKeyField with multiple subtests covering federation scenarios where entity key fields exist exclusively in ExternalFieldNames.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Possibly related PRs

• fix: requires is not ignored when all fields provided #1380: Modifies federation key/provides handling in the same planner files (key_fields_visitor.go and datasource_filter_collect_nodes_visitor.go).
• fix: planner fixes for abstract selection rewriter, node selection rules #1360: Updates planner node-selection logic in visitor/selection rules related to key-field and datasource node handling.

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix: query planning for shared entities' is concise and directly addresses the main objective of the PR, which is fixing query planning failures for shared entities with external key fields.

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✨ Finishing Touches

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[StarpTech]

Close it because it's a composition error.