Flatten Parent child collection using mongodb

Question

I need to flatten my parent child collection in mongodb . This is similar to question asked for sql at :

Flatten parent child hierarchy with multiple parents

my collection is like category: {_id, type, name, parent:(mapped to self i.e. category collection) )}

And the current depth of child is 3 with type L1, L2,L3

Result will be with fields: (L1_id, L1_name, L2_id, L2_name, L3_id, L3_name)

Kindly help

Answer 1

You can use mongodb aggregation pipeline to achieve the same. More specifically you can use $lookup twice to populate parent and its parent, and finally $project to flatten the structure.

Try this:

Category.aggregation([{
    $lookup : {
        from :"categories",
        localField : "parent",
        foreignField : "_id",
        as  :"parent"
    }
},{
    $unwind : "$parent"
},{
    $lookup : {
        from :"categories",
        localField : "parent.parent",
        foreignField : "_id",
        as  :"parent.parent"
    }
},{
    $unwind : "$parent.parent"
},{
    $project : {
        l1_id  : "$_id",
        l1_name : "$name",
        l2_id : "$parent._id", 
        l2_name : "$parent.name" ,
        l3_id : "$parent.parent._id", 
        l2_name : "$parent.parent.name" 
    }
}]).then(result => {
    // result will have l1_id, l1_name, l2_id, l2_name, l3_id, l3_name
    // where l2 is the parent,
    // and l3 is the parent of parent 
}).

Note: $unwind is used after $lookup stage, as $lookup returns an array, we need to unwind it to convert it to object.

For more info please read Mongodb $lookup documentation,and $project documentation.

i hope this helps you out.