Fetch data from one collection and add in another collection

Here are two models User and Item

type User struct {
    ID        primitive.ObjectID   `json:"id" bson:"_id,omitempty"`
    Email     string        `json:"email,omitempty" bson:"email,omitempty"`
    FirstName string        `json:"firstName,omitempty" bson:"firstName,omitempty"`
    LastName  string        `json:"lastName,omitempty" bson:"lastName,omitempty"`
    Password  string        `json:"password,omitempty" bson:"password,omitempty"`
    Item      Item          `json:"item,omitempty" bson:"item,omitempty"`

type Item struct {
    ID          primitive.ObjectID `json:"id,omitempty" bson:"_id,omitempty"`
    Category    string             `json:"category,omitempty" bson:"category,omitempty"`
    Name        string             `json:"name,omitempty" bson:"name,omitempty"`
    Description string             `json:"description,omitempty" bson:"description,omitempty"`
    Price       int                `json:"price,omitempty" bson:"price,omitempty"`

I have inserted many samples into Item collection.
But I don’t have any Idea how to insert data from Item collection into User collection.