How to do inner joining in MongoDB?

Question

Is it possible to do SQL inner joins kind of stuff in MongoDB , well i know there is

$lookup

attribute in aggregation pipeline and

Answer 1

> show dbs
admin   0.000GB
config  0.000GB
local   0.002GB
> use local
switched to db local
> show collections
startup_log
test1
test2
> db.test2.aggregate([{
...    $lookup: {
...       from: "test1",
...       localField: "id",
...       foreignField: "id",
...       as: "aggTest"
...    }
... }])

Answer 2

I found answer my self it was

$unwind done the trick to me following query worked for me

    db.USER.aggregate([{
            $lookup: {
                from: "USER_ROLE",
                localField: "ID",
                foreignField: "USER_ID",
                as: "userRole"
            }
        }, {
            $unwind: {
                path: "$userRole",
                preserveNullAndEmptyArrays: false
            }
        }, {
            $lookup: {
                from: "ROLE",
                localField: "userRole.ROLE_ID",
                foreignField: "ID",
                as: "role"
            }
        }, {
            $unwind: {
                path: "$role",
                preserveNullAndEmptyArrays: false
            }
        }, {
            $match: {
                "role.ROLE_NAME": "staff"
            }, {
                $project: {
                    USER_NAME: 1,
                    _id: 0
                }
            }
            ]).pretty()

Anyway thanks for the answers

Answer 3

As Tiramisu wrote this looks like schema issue.

You can make a manual inner join, by removing documents where $lookup returned empty array.

....
{$lookup... as myArray},
{$match: {"myArray":{$ne:[]}}},
{$lookup... as myArray2},
{$match: {"myArray2":{$ne:[]}}},

schema change

I personally will go for schema update, like this:

db.User.find({})
{
   ID : 1,
   USER_NAME : "John",
   password : "pass"
   roles:[{ID : 1,  ROLE_NAME : "admin"}]
}


db.ROLE.find({})
{
   ID : 1,
   ROLE_NAME : "admin"
},

Answer 4

Will this help

const RolesSchema = new Schema({
  ....

});
const Roles = mongoose.model('Roles', RolesSchema);


const UserSchema = new Schema({
  ...

  roles: [{ type: mongoose.Schema.Types.ObjectId, ref: "Roles" }]
});

using the populate on userschema you can also reduce the redundancy