MongoDB push to nested array using Linq expressions

Question

To execute a push using de MongoDB C# Driver, I need to instantiate a FieldDefinition<MyMongoDocumentType, MyNestedArrayType[]>.

I know that I can instantiate this FieldDefinition using strings...

FieldDefinition<MyMongoDocumentType, NestedArrType[]> field = "MyArray.$.MyNestedArray";

I tried the same using Linq expressions, like this:

FieldDefinition<MyMongoDocumentType, NestedArrType[]> field =
    new ExpressionFieldDefinition<MyMongoDocumentType, NestedArrType[]>(
        doc => doc.MyArray.First().MyNestedArray
    );

But I got this error:

System.InvalidOperationException: Unable to determine the serialization information for doc => doc.MyArray.First().MyNestedArray.

Is there any way to create a FieldDefinition of a nested array using Linq expression that works?

mickl · Accepted Answer · 2019-01-17 21:47:34Z

1

You can use -1 as an array index to represent positional operator ($):

FieldDefinition<MyMongoDocumentType, NestedArrType[]> field =
            new ExpressionFieldDefinition<MyMongoDocumentType, NestedArrType[]>(
                doc => doc.MyArray[-1].MyNestedArray
            );

To make it work you also need additional query condition on MyArray which can be done using ElemMatch in MongoDB .NET driver, for instance:

Builders<MyMongoDocumentType>.Filter.ElemMatch(x => x.MyArray, f => f.NestedId == 1);

answered Jan 17, 2019 at 21:47

mickl

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1 Comment

lmcarreiro Over a year ago

It works, thank you! I recommend using #pragma warning disable CS0251 at the beginning of the file and #pragma warning restore CS0251 at the end, to avoid the warning on the negative index.

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