fix(graphile-build-pg): add computed fields to polymorphic interfaces - #3216

Addresses #1951.

A function that takes a polymorphic table's row (a computed column) is not added to the table's interface, for mode:single and mode:relational. For mode:relational it is not added to the implementations either, since the function takes the root's row type, not an implementation's. PgCustomTypeFieldPlugin has // TODO: remove this and fix issue where computed columns are not added to polymorphic types and // TODO: factor in where computed column is added to mode:relational base (shared) table.

Visible in the existing polymorphic test schema: single_table_items_meaning_of_life is on the single-table implementations but not on SingleTableItem; relational_items_meaning_of_life is on no type.

{ allRelationalItems { nodes { __typename meaningOfLife } } }
# Cannot query field "meaningOfLife" on type "RelationalItem".
{ allRelationalPosts { nodes { meaningOfLife } } }
# Cannot query field "meaningOfLife" on type "RelationalPost".

Change (PgCustomTypeFieldPlugin)

• modFields no longer returns early for single / relational interfaces (union interfaces still do).
• The computed sources of an implementation include those of the relational roots it implements, found through codec.extensions.relationalInterfaceCodecName (set by PgPolymorphismPlugin), at any depth.
• On an implementation, the field's plan first follows the relation(s) from the implementation's row to the owning root's row, only when they differ; otherwise the plan is unchanged, so no existing computed-column plan changes.

Performance: on a relational implementation the function is evaluated from the root row reached by a relation. Non-polymorphic tables are unchanged.

#3066 (@applyToType for computed columns) touches the same file; the two are complementary and one will need a rebase.

Commits

1. test(graphile-build-pg): adds polymorphic.relational_items_type_label(relational_items) to the fixture, __tests__/queries/polymorphic/computed-fields.test.graphql with snapshots (fields read through the relational interface, a relational implementation and the single-table interface; values checked against the root row), and the expected updates of four schema snapshots (meaningOfLife and typeLabel on both interfaces and on the five relational implementations). On main:

   ● expect(errors).toBeFalsy()
     Invalid operation document: Cannot query field "meaningOfLife" on type "RelationalItem".
   ● prints a schema for polymorphic
     - Expected  - 13
     + Received  +  0
   Test Suites: 5 failed, 5 total
   Tests: 16 failed, 16 total
   

2. fix(graphile-build-pg): the change above + changeset. Test Suites: 5 passed, 5 total; Tests: 16 passed, 16 total.

Verification

Run on this branch against PostgreSQL 17 (UTF8): postgraphile/postgraphile full jest: 236 passed, 236 total, 804 passed, 804 total; yarn test:schema-exports: passes; EXPORT_SCHEMA=typeDefs jest __tests__/queries __tests__/mutations: 730 passed, 730 total; EXPORT_SCHEMA=graphql-js jest __tests__/queries/polymorphic __tests__/queries/refs: 163 passed, 163 total; graphile-build-pg yarn test: passes. prettier --check and eslint on the changed files: clean. Baseline on main with the same setup: 235 suites / 792 tests, typeDefs 718, graphql-js 151, all passing.