Is it possible to compare string with ObjectId via $lookup

问题

table1 has a field string "value" and table2 has a field "value" as ObjectId, Is it possible to do a query like this or how to write

table1.aggregate([
    { 
        $lookup: { 
            from: "table2", 
            localField: "value", 
            foreignField: "_id", 
            as: "test" 
        }  
    }
])

回答1:

As far I know to join collections using $lookup operator in MongoDB data type should be same. If type mismatch then $lookup will not work. So to join you should use those field that are same type because it check equality.

The $lookup stage does an equality match between a field from the input documents with a field from the documents of the “joined” collection

• If localField type object then foreignField should be object

• If localField type string then foreignField should be string

• If localField type number then foreignField should be number

$lookup Documentation

来源：https://stackoverflow.com/questions/41393899/is-it-possible-to-compare-string-with-objectid-via-lookup