Query: support composing over GroupBy + First per group (navigations and joins)

[ducmerida]

Fixes #38686
Fixes #28125

Summary

Composing over the entity produced by a top-1-per-group query failed in two independent layers:

ctx.TimeSheets
    .GroupBy(t => t.OrderId)
    .Select(g => g.OrderBy(x => x.Id).First())
    .OrderBy(t => t.Order.Number)   // ← "Translation of member 'Order' ... failed" (misleading: it IS mapped)

1. Navigation expansion built the result tree from Expression.Default, so the group element entity lost its entity reference and no navigation on it could be expanded (Query: Re-evaluate concept of pending selector in nav expansion #20291 territory).
2. Even with the navigation expanded (or with a manual join, no navigations involved — The LINQ expression 'ProjectionBindingExpression: 0/1/2' could not be translated. #28125), the relational projection binder had no handling for a client projection slot holding a single-result subquery; its lowering only ran at the end of translation in ApplyProjection — "we don't allow this terms to compose (yet!)".

Implementation

• NavigationExpandingExpressionVisitor.ProcessSelect(GroupBy…) now snapshots the selector structure before reducing (same mechanism ProcessDistinct uses for its pending selector), so a cardinality-reduced group element keeps its entity reference and later navigation accesses expand into regular joins over the grouped query.
• The single-result to-one lowering is extracted from ApplyProjection into a shared core (LowerSingleResultSubqueryCore, parameterized on the projection sink — behavior-identical for the existing deferred path) and the projection binder now invokes it on demand when a composed query binds a single-result slot: pushdown if needed (remapping the binder's already-collected projections with the pushdown's remapping visitor), prunable to-one OUTER APPLY, shaper remap. Since the lowering mutates the select expression, the binder memoizes the lowered shaper per projection slot (same idiom as its existing _projectionBindingCache), so a projection referencing the slot more than once reuses the single join.

Scalar member access after the per-group First() already worked (#26748 fixed the second-projection case; OrderBy by scalar translates as a correlated scalar subquery). This PR adds the missing entity-level composition. Known follow-up: the join key over the group element still binds as a correlated scalar subquery in the ON clause instead of reusing the lowered join's column — correct but suboptimal (the duplication described in #20291).

All currently-failing shapes throw today, so no existing SQL baselines change.

Aware of #37859/#32957 (moving navigation expansion into translation, 12.0): the navigation-expansion piece is a targeted fix for the 11 pipeline; the on-demand lowering lives on the translation side, aligned with that direction, and the new tests pin the behavior for the rewrite.

Testing

• New specification tests following the GroupBy_Select_Entire_Entity family: navigation OrderBy/Where/projection over the group element, a manual Join over it (The LINQ expression 'ProjectionBindingExpression: 0/1/2' could not be translated. #28125), and a projection referencing the group element twice (whose baseline pins the single-join shape). Results-verified baselines on SQL Server; skipped on InMemory like the rest of the family (When using GroupBy, getting "the given key 'EmptyProjectionMember' was not present in the dictionary" #31209).
• Full local runs: EFCore.SqlServer.FunctionalTests (50,818), EFCore.Sqlite.FunctionalTests (38,375), EFCore.InMemory.FunctionalTests (28,230) — remaining failures are environment-only (Full-Text Search, memory-optimized, SqlAzure, PrimitiveCollections BCL-preview artifact); InMemory fully clean.

[Copilot]

Pull request overview

This PR fixes translation failures when composing further query operators over the entity produced by a “top-1-per-group” pattern (GroupBy(...).Select(g => g.OrderBy(...).First()/FirstOrDefault())), especially when later traversing navigations or performing joins. It does so by preserving entity shape through navigation expansion and by enabling on-demand lowering of single-result subqueries during relational projection binding.

Changes:

• Navigation expansion now snapshots selector structure before reducing cardinality, preserving entity references for later navigation expansion.
• Relational SelectExpression extracts/reuses the single-result subquery lowering logic and exposes an internal on-demand lowering hook used by the projection binder when a single-result subquery is composed over.
• Adds new spec + SQL Server functional tests for navigation OrderBy/Where/projection and a manual Join over the per-group First() element; skips these in InMemory consistently with the existing family.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/EFCore.SqlServer.FunctionalTests/Query/NorthwindGroupByQuerySqlServerTest.cs	Adds SQL assertions for the new GroupBy+First-per-group composition shapes on SQL Server.
test/EFCore.Specification.Tests/Query/NorthwindGroupByQueryTestBase.cs	Adds new cross-provider specification tests covering navigation and join composition over the grouped “first” entity.
test/EFCore.InMemory.FunctionalTests/Query/NorthwindGroupByQueryInMemoryTest.cs	Skips the new tests for InMemory in line with existing GroupBy/entire-entity limitations.
src/EFCore/Query/Internal/NavigationExpandingExpressionVisitor.cs	Preserves selector structure across cardinality reduction to keep entity references for later navigation expansion.
src/EFCore.Relational/Query/SqlExpressions/SelectExpression.cs	Refactors single-result subquery lowering into a shared core and adds an internal on-demand lowering entry point for projection binding.
src/EFCore.Relational/Query/Internal/RelationalProjectionBindingExpressionVisitor.cs	Invokes on-demand lowering when binding a single-result subquery projection so composed queries can continue translating.