Mongodb: lookup with field as array of objects

Question

In my collection, I am referring field as an array of object(skill and year are the keys ) in which one field in that object refers to another collection

primary_skills: [
    {
      skill_id: {
        type: mongoose.Schema.Types.ObjectId,
        ref: 'Skill'
      },
      years: Number
    }
  ],

What I want to achieve to list all skills

profile.aggregate([{
     $lookup:{
        from:'skills',
        localField:'primary_skills.skill_id',
        foreignField:'_id',
        'as':'primary_skills'
      }
   
}])

This works fine but the year key in the object is missing while running this

Output:

"primary_skills": [
            {
                "_id": "5f0d885307a96c3040b3667b",
                "name": "PHP",
                "__v": 0
            }
        ]

Expected output:

"primary_skills": [
            {
                "name": "PHP",
                "Years":"12"  
            }
        ]