Method LeftJoin

Namespace

MongoDB.Driver.Linq

Assembly

MongoDB.Driver.dll

LeftJoin<TOuter, TInner, TKey, TResult>(IQueryable<TOuter>, IQueryable<TInner>, Expression<Func<TOuter, TKey>>, Expression<Func<TInner, TKey>>, Expression<Func<TOuter, TInner, TResult>>)

Correlates the elements of two sequences based on matching keys, preserving all outer elements even when no matching inner element exists (left outer join semantics).

public static 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)

Parameters

outer IQueryable<TOuter>

The first sequence to join.

inner IQueryable<TInner>

The sequence to join to the first sequence.

outerKeySelector Expression<Func<TOuter, TKey>>

A function to extract the join key from each element of the first sequence.

innerKeySelector Expression<Func<TInner, TKey>>

A function to extract the join key from each element of the second sequence.

resultSelector Expression<Func<TOuter, TInner, TResult>>

A function to create a result element from an outer element and a matching inner element (or default if no match).

Returns

IQueryable<TResult>

An IQueryable<T> that contains elements of type TResult obtained by performing a left outer join on two sequences.

Type Parameters

TOuter

The type of the elements of the first sequence.

TInner

The type of the elements of the second sequence.

TKey

The type of the keys returned by the key selector functions.

TResult

The type of the result elements.

Remarks

When inner carries additional query operators (such as Where) it is translated to a $lookup that combines localField/foreignField with a pipeline, which requires MongoDB 5.0 or later.