Mongoose deep populating multiple 2nd level objects not working

I have a model A wit this field :

var field = {
    foo: String,
    b: [{ type: Schema.Types.ObjectId, ref: 'B' }]
}  

and model B with this feilds :

var field = {
    c: { type: Schema.Types.ObjectId, ref: 'C' } // let's say this has 3 string field
    d: { type: Schema.Types.ObjectId, ref: 'D' } // so was this
}

Based on this answer by Trinh Hoang Nhu by doing this-

 A.find({_id:req.params.id})
    .populate({ path: 'patient', model: Patient,
        populate: {
            path: 'b',
            model: B
        },
        populate: {
            path: 'c',
            model: C
        },
    })
    .exec(function (err, doc) {
        res.send(doc);
    })

-it should return the populated 2nd level and it did but the problem is it only populate the last path declared on the .populate({}) function, in this case only the model C is populated. But when you changed it's position with model B then model B will be populated.

The query above returns something like this :

[
    {
        "foo":"Bar",
        "b": [
            {
                "c":"a32s1da4fas1a23s1da56s4c",
                "d":{
                    "foo1":"Bar1",
                    "foo2":"Bar2",
                    "foo3":"Bar3"
                }
            },
            {
                "c":"a32s1da4fas1a23s1da56s4d",
                "d":{
                    "foo1":"Bar1",
                    "foo2":"Bar2",
                    "foo3":"Bar3"
                }
            }
            // so on ...
        ]
    }
]

I was expecting something like this :

[
    {
        "foo":"Bar",
        "b": [
            {
                "c":{
                    "foo1":"Bar1",
                    "foo2":"Bar2",
                    "foo3":"Bar3"
                },
                "d":{
                    "foo1":"Bar1",
                    "foo2":"Bar2",
                    "foo3":"Bar3"
                }
            },
            {
                "c":{
                    "foo1":"Bar1",
                    "foo2":"Bar2",
                    "foo3":"Bar3"
                },
                "d":{
                    "foo1":"Bar1",
                    "foo2":"Bar2",
                    "foo3":"Bar3"
                }
            }
            // so on ...
        ]
    }
]