I configuring an api similar to the blog category.

The object ID of the category is recorded for each post.

Please help us with how we can extract data in the form of the third image.

• first code: category data
• second code : post data ( category object id value )
• third code : the result that I want

{ // category
    _id: '61dd8eb5368930c3a6cee140',
    name: 'parent',
    childern: [
        { _id: '61dd8eb5368930c3a6cee141',
          name: 'child1' },
        { _id: '61dd8eb5368930c3a6cee142',
          name: 'child2' }
    ]
}

first code

{ // post data
    _id: '61dd8eb5368930c3a6cee160',
    name: 'first post - parent',
    category: '61dd8eb5368930c3a6cee140',
},
{
    _id: '61dd8eb5368930c3a6cee161',
    name: 'second post - child',
    category: '61dd8eb5368930c3a6cee141',
},

second code

{ // the result that I want
    _id: '61dd8eb5368930c3a6cee160',
    name: 'first post - parent',
    category: {
        _id: '61dd8eb5368930c3a6cee140',
        name: 'parent'
    }
},
{
    _id: '61dd8eb5368930c3a6cee161',
    name: 'second post - child',
    category: {
        _id: '61dd8eb5368930c3a6cee141',
        name: 'child1'
    }
},

third code == i want it

pleas help me