[ajcvickers]

db.Set().Select(e => e.OwnedCollection.Count) threw ArgumentException in every case (bare and arithmetic-wrapped alike), because EF Core lowers a collection-navigation Count to Queryable.Count(Queryable.AsQueryable(source)) even for an in-document embedded List, and
MongoProjectionBindingExpressionVisitor had no case for that shape - it fell into the generic fallback, which tried to rebuild the Queryable.Count call with the already-materialized List, which isn't an IQueryable.

Add TryBindEmbeddedCollectionNavigationCount to recognize the shape and rewrite it as a null-safe Enumerable.Count/LongCount over the navigation's CollectionShaperExpression (null-safe because a missing/explicitly-null stored array materializes to null rather than an empty collection).

[Copilot]

Pull request overview

Fixes a query translation/binding gap in the MongoDB EF Core provider where projecting OwnedCollection.Count (or LongCount) over an embedded owned collection could throw ArgumentException due to EF Core lowering the expression into a Queryable.Count(Queryable.AsQueryable(source)) shape that doesn’t match the provider’s fallback binding logic.

Changes:

• Add a dedicated binding path in MongoProjectionBindingExpressionVisitor to detect embedded-navigation Count/LongCount lowered as Queryable.*(Queryable.AsQueryable(...)) and rewrite to a null-safe Enumerable.Count/LongCount over the CollectionShaperExpression.
• Add functional tests to cover bare Count/LongCount projections and arithmetic-wrapped Count over an owned embedded collection, including null/empty cases.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/MongoDB.EntityFrameworkCore.FunctionalTests/Query/OwnedEntityTests.cs	Adds regression tests for Count/LongCount projections over embedded owned collections (bare and wrapped).
src/MongoDB.EntityFrameworkCore/Query/Visitors/MongoProjectionBindingExpressionVisitor.cs	Adds embedded-collection Count/LongCount binding logic and null-safe helpers to avoid Queryable.Count rebuild failures.

Suppressed comments (1)

src/MongoDB.EntityFrameworkCore/Query/Visitors/MongoProjectionBindingExpressionVisitor.cs:763

• NullSafeLongCount is intended to tolerate a null embedded collection (mirroring NullSafeCount), but its parameter is declared non-nullable IEnumerable even though the implementation treats it as nullable. Make the parameter nullable so the signature matches behavior and intent under enable.

    private static long NullSafeLongCount<T>(IEnumerable<T> source)
        => source?.LongCount() ?? 0;

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[ajcvickers]

Superseded by #339