VB net & Mongo: Using where clause with LINQ causes error "Unsupported where clause: (Boolean)Operators.CompareObjectLess" - vb.net

Question

I have a collection in MongoDB and I'm using MongoDB Driver in VB net. I want to update several documents depending on a condition.
For this, I want to use LINQ, but the select causes an error and I don't know how to fix it.
Here's the code:
Dim update_for As UpdateBuilder
Dim query_for As IMongoQuery
Dim coll_for = db.GetCollection(Of MyClass)("collection_1")
Dim queryMun = (From a In coll_for _
Where (a.field_1 < 10000) _
Select a)
For Each emp In queryMun
query_for = Query.EQ("_id", emp.Id)
update_for = Update.Set("field_1", BsonValue.Create("0" + emp.field_1))
coll.Update(query_for, update_for, opts)
Next
When it executes de For Each sentence, it raises the exception: Unsupported where clause: (Boolean)Operators.CompareObjectLess(a.field_1, 10000, true).
What am I doing wrong?
Thank you very much for your help.

Answer 1

I think the error is clear:
You can't use a Less Than "<" operator in you WHERE clause because it's unsupported.

Answer 2

I have found a way to do this update based on the value of the attribute itself. What I want to do is add a "0" at the beginning of the attribute value, for example, if field_1=4567, after the update field_1='04567'.
Here's the code:
Dim update_for As UpdateBuilder
Dim query_for As IMongoQuery
Dim opts = New MongoUpdateOptions
opts.Flags = UpdateFlags.Multi
Dim coll_for = db.GetCollection(Of MyLINQClass)("collection_1")
Dim queryMun2 As New QueryDocument
Dim query_1 = Query.LT("field_1", MongoDB.Bson.BsonValue.Create(10000))
queryMun2.AddRange(query_1.ToBsonDocument)
Dim queryMun = coll_for.Find(queryMun2)
For Each emp In queryMun
query_for = Query.EQ("_id", emp.Id)
update_for = Update.Set("field_1", BsonValue.Create("0" + emp.FField_1.ToString))
coll.Update(query_for, update_for, opts)
Next
And here is the definition of MyLINQClass:
Public Class MyLINQClass
<BsonElementAttribute("_id")> _
Public Property Id() As ObjectId
<BsonElementAttribute("field_1")> _
Public Property FField_1() As Object
End Class