Can not get mongodb Aggregate query result in c#

Question

I have create two data models and try to apply different query on it using mongoDb C# driver. Here is the query in a get method

var res = _postCollection.Aggregate().Lookup("_commentCollection", "Comments", "_id", "result").ToList();
return OK(res)

But when I call the method from postman get the following result

System.InvalidCastException: Unable to cast object of type 'MongoDB.Bson.BsonObjectId' to type 'MongoDB.Bson.BsonBoolean'.
   at get_AsBoolean(Object )
   at System.Text.Json.JsonPropertyInfoNotNullable`4.OnWrite(WriteStackFrame& current, Utf8JsonWriter writer)
...............

score 0 · Answer 1 · answered Jan 20 '21 at 07:38

0

Your problem is casting! You want to try cast the object type "MongoDB.Bson.BsonObjectId" to type "MongoDB.Bson.BsonBoolean". This is not possible and you will receive an exception

answered Jan 20 '21 at 07:38

ABlue

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I don't know where MongoDB.Bson.BsonBoolean come from , I didn't use this type – Hridoy_089 Jan 20 '21 at 07:43

score 0 · Answer 2 · answered Jan 20 '21 at 08:52

0

The parameter for the Lookup are Ref

string foreignCollectionName,
FieldDefinition<TResult> localField,
FieldDefinition<TForeignDocument> foreignField,
FieldDefinition<TNewResult> as,
AggregateLookupOptions<TForeignDocument, TNewResult> options

so the localField and foreignField must be of the same type. In your case they are of different type clearly shown in the error

System.InvalidCastException: Unable to cast object of type 'MongoDB.Bson.BsonObjectId' to type 'MongoDB.Bson.BsonBoolean'.
   at get_AsBoolean(Object )
   at System.Text.Json.JsonPropertyInfoNotNullable`4.OnWrite(WriteStackFrame& current, Utf8JsonWriter writer)

Here is the similar question on SO which can help you.

answered Jan 20 '21 at 08:52

शेखर

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Both was same type, I tested on mongo shell there it worked but not here – Hridoy_089 Jan 20 '21 at 09:12
@Hridoy_089 and the type is? – शेखर Jan 20 '21 at 09:30
type BsonObjectId – Hridoy_089 Jan 20 '21 at 09:40
@Hridoy_089 the default name for this column is _id have you changed for the second collection? – शेखर Jan 20 '21 at 09:56
I used public ObjectId Id { get; set; } for both collections – Hridoy_089 Jan 20 '21 at 10:01
@Hridoy_089 are your sure the type of "Comments" "_id" are same? You are trying to join two collection and getting the result? – शेखर Jan 20 '21 at 10:15
Comments is list of ids. but I tried with another item string vs string but getting same error – Hridoy_089 Jan 20 '21 at 10:17
@Hridoy_089 I would request to see aggrigation pipelie of mongo db and search for lookup. Here is a link https://www.c-sharpcorner.com/article/aggregation-pipeline-in-mongodb/ https://stackoverflow.com/questions/50530363/aggregate-lookup-with-c-sharp – शेखर Jan 20 '21 at 10:25

Jerson Uyekita · Answer 3 · 2022-12-27T03:16:06.077

0

You forgot to set the generics to Lookup()

var res = _postCollection.Aggregate().Lookup<ResultType, Comment>("_commentCollection", "Comments", "_id", "result").ToList();

edited Dec 27 '22 at 03:16

answered Dec 27 '22 at 03:15

Jerson Uyekita

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Can not get mongodb Aggregate query result in c#

3 Answers3