how to auto update mongodb field

Question

I have a Category schema and I want when children field changes to check its length and if the length is equal to 0 then has_children should be false and the opposite should apply.

This is my model :

const category_item = new mongoose.Schema({

   //...other fields...,
   children: { type: Array, required: true, default: [] }
   has_children:  // if children length equal to 0 get false and opposite     
});

Answer 1

keep

const category_item = new mongoose.Schema({

    ...anotherFields...,
    children : {type : Array , required : true , default : []}
    has_children : {type: Boolean}
});

and add a pre save hook

category_item.pre('save', function(next) {
    if (this.children && this.children.length > 0) {
        this.has_children = true;
    } else {
        this.has_children = false;
    }
    next();
});

Reference: http://mongoosejs.com/docs/middleware.html