Fix FirstOrDefaultAsync(x => x.Id == value) always returning null on DocumentCollection<string, T>

[Copilot]

FirstOrDefaultAsync(x => x.Id == value) on a DocumentCollection<string, T> always returned null even when the document existed. The ToLowerInvariant() workaround succeeded because it bypassed both the IndexOptimizer and BsonExpressionEvaluator, falling through to FindAllAsync() + in-memory LINQ filter.

Root cause

Two gaps in the query expression pipeline:

• IndexOptimizer.Matches compared secondary index PropertyPaths[0] against the LINQ-extracted C# property name using plain OrdinalIgnoreCase. "_id" (BSON wire name) and "Id" (C# property name) are not equal under that comparison, so any index stored under the BSON alias would never be selected.

• BsonExpressionEvaluator.TryCompileBody only normalised "id" → "_id" when building the BSON scan predicate, leaving the "_id" spelling itself unhandled.

Changes

• IndexOptimizer.Matches — normalises both sides through "_id" ↔ "Id" before comparing, so a secondary index is found regardless of which spelling was used when it was registered:

static string Normalize(string s) =>
    s.Equals("_id", StringComparison.OrdinalIgnoreCase) ? "Id" : s;

return Normalize(indexPath).Equals(Normalize(propertyName), StringComparison.OrdinalIgnoreCase);

• BsonExpressionEvaluator.TryCompileBody — maps both "id" and "_id" to "_id" so the BSON scan predicate always targets the correct wire field name.

• StringMapperIdLookupTests — 7 regression tests covering constant equality, closure-captured variable equality, with and without a secondary unique index on Id, the ToLowerInvariant() workaround, multi-document filtering, and no-match returning null.

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Pull request overview

Fixes a LINQ query regression where FirstOrDefaultAsync(x => x.Id == value) could return null for DocumentCollection<string, T> when a secondary index on the Id field exists, by aligning primary-key field naming between the query optimizer and BSON predicate compilation.

Changes:

• Normalize Id vs _id when IndexOptimizer matches a predicate to an available secondary index.
• Normalize id/_id to _id in BsonExpressionEvaluator when compiling simple binary predicates.
• Add regression tests covering constant/closure equality, with/without a secondary unique Id index, workaround behavior, multi-document filtering, and no-match cases.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
tests/BLite.Tests/StringMapperIdLookupTests.cs	Adds regression coverage for string primary-key equality queries across optimized/non-optimized paths.
src/BLite.Core/Query/IndexOptimizer.cs	Makes index selection robust to Id vs _id naming differences.
src/BLite.Core/Query/BsonExpressionEvaluator.cs	Ensures primary-key field normalization in compiled BSON scan predicates for binary comparisons.

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[Copilot]

In TryCompileBody’s simple-binary branch, the Id/_id normalization logic is now duplicated and slightly diverges from the earlier .Equals()-method-call branch (which still only special-cases "id"). Consider extracting a small helper (e.g., NormalizePrimaryKeyField) and using it in both branches; also, the added comment mentions nested-entity behavior even though this evaluator returns null for nested paths (see TryCompile_NestedPath_ReturnsNull), which can be misleading here.