.NET 10 introduces a first-class LeftJoin operator on Queryable that enables all sorts of include and join scenarios we want to unlock not just in the driver but also in our EF Provider.

  IQueryable<TResult> LeftJoin<TOuter, TInner, TKey, TResult>(
      this IQueryable<TOuter> outer,
      IQueryable<TInner> inner,
      Expression<Func<TOuter, TKey>> outerKeySelector,
      Expression<Func<TInner, TKey>> innerKeySelector,
      Expression<Func<TOuter, TInner, TResult>> resultSelector)

A generated pipeline for a simple left join + select looks like this:

{{  { "$project":

{ "_outer": "$$ROOT", "_id": 0 }

}}}
{{  { "$lookup":

{ "from": "customers", "localField": "_outer.CustomerId", "foreignField": "_id", "as": "_inner" }

}}}
{{  { "$unwind":

{ "path": "$_inner", "preserveNullAndEmptyArrays": true }

}}}
{{  { "$project":

{ "Outer": "$_outer", "Inner": "$_inner", "_id": 0 }

}}}

Supported patterns:

• Bare LeftJoin with LeftJoinResult<TOuter, TInner> result type (Include / navigation property case)
• LeftJoin followed by .Select(), .Where(), or another .LeftJoin() (mid-pipeline chaining for ThenInclude)

Required by the MongoDB EF Core provider to translate EF Core Include/ThenInclude navigation-property expressions without rebuilding expression trees in the provider itself.

Cross-platform compatibility: On .NET 10+, System.Linq.Queryable.LeftJoin exists natively. To support this pattern on prior versions of .NET we'll also include a MongoQueryable.LeftJoin with the same signature so our queries work across all supported versions of .NET.