Haskell MongoDB查询中如何使用“ $ and”运算符

Question

I am trying to use "$and" operator in Haskell mongoDB which is not working:

myFilter = do
   cursor <- MongoDB.find (MongoDB.select ["$and" =: [["field1" =: "test1"], ["field2" =: "test2"], ["field3" =: "test3"]]],  "db") 
   rest cursor

Thanks in advance.

Answer 1

You have an unnecessary comma before "db" which is causing a type error.

Try this:

myFilter = do
   cursor <- MongoDB.find (MongoDB.select ["$and" =: [["field1" =: "test1"], ["field2" =: "test2"], ["field3" =: "test3"]]]  "db")
   rest cursor

Note:

• you don't really need to use "$and" because it is the default.
• where you specify "db" is the name of a collection.