MongoDB .Net driver 2.0 Builders Filter (field to array comparison) - mongodb

Question

I need to get all usernames from "followingList.username" and compare with
posts' usernames, if there any match need to add that one to an array.
Person Model
{
"_id" : ObjectId("554f20f5c90d3c7ed42303e1"),
"username" : "fatihyildizhan",
"followingList" : [
{
"_id" : ObjectId("55505b6ca515860cbcf7901d"),
"username" : "gumusluk",
"avatar" : "avatar.png"
},
{
"_id" : ObjectId("58505b6ca515860cbcf7901d"),
"username" : "yalikavak",
"avatar" : "avatar.png"
},
{
"_id" : ObjectId("58305b6ca515860cbcf7901d"),
"username" : "gumbet",
"avatar" : "avatar.png"
}
]
}
Post Model
{
"_id" : ObjectId("554f2df2a388R4b425b89833"),
"username" : "yalikavak",
"category" : "Summer",
"text" : "blue voyage with yacht"
},
{
"_id" : ObjectId("554f2df2a388P4b425b89833"),
"username" : "yalikavak",
"category" : "Winter",
"text" : "is coming ..."
},
{
"_id" : ObjectId("554f2df2a388K4b425b89833"),
"username" : "gumbet",
"category" : "Fall",
"text" : "there are many trees"
}
I try to get result code block as below but couldn't figure it out.
var filter = Builders<Post>.Filter.AnyEq("username", usernameList);
var result = collection.Find(filter).ToListAsync().Result;
Can you help me with this?
Thanks.

Answer 1

The logic is flipped, what you need is an $in query if I understand your use case correctly:
var filter = Builders<Post>.Filter.In("username", usernameList);
var result = collection.Find(filter).ToListAsync().Result;
In your case, username is a simple field and you want to match against a list of candidates. AnyEq is used to check that, from an embedded list of complex objects, at least one matches a criterion. That still translates to a simple query in MongoDB, but requires to 'reach into' the object which needs a more complicated syntax.