aggregation framework - Recursive search on a collection in MongoDB

Question

I have a list of documents in MongoDB with tree structure, where Model Tree Structures with Parent References pattern used. I want a single aggregation query which returns ancestor list(till the root), given the 'name' property.

Structure:

{
  '_id': '1',
  'name': 'A',
  'parent': '',
},
{
  '_id': '2',
  'name': 'B',
  'parent': 'A',
},
{
  '_id': '3',
  'name': 'C',
  'parent': 'B',
},
{
  '_id': '4',
  'name': 'D',
  'parent': 'C',
}

Aggregation result:(Given, name = 'D')

{
  '_id': '4',
  'name': 'D',
  'ancestors': [{name:'C'}, {name:'B'}, {name:'A'}]
}

Note: I can't change the document structure now. It will cause many problems. I saw many solutions which suggest to use Model Tree Structures with an Array of Ancestors. But I cannot use it now. Is there any way to achieve it with the above pattern using single aggregation query? Thank you

Answer 1

Starting from MongoDB 3.4, we can do this with the Aggregation Framework.

The first and most important stage in our pipeline is the $graphLookup stage. $graphLookup allows us to recursively match on the "parent" and "name" field. As result, we get the ancestors of each "name".

The next stage in the pipeline is the $match stage where we simply select the "name" we are interested in.

The final stage is the $addFields or $project stage where we apply an expression to the "ancestors" array using the $map array operator.

Of course with the $reverseArray operator we reverse our array in order to get the expected result.

db.collection.aggregate(
    [ 
        { "$graphLookup": { 
            "from": "collection", 
            "startWith": "$parent", 
            "connectFromField": "parent", 
            "connectToField": "name", 
            "as": "ancestors"
        }}, 
        { "$match": { "name": "D" } }, 
        { "$addFields": { 
            "ancestors": { 
                "$reverseArray": { 
                    "$map": { 
                        "input": "$ancestors", 
                        "as": "t", 
                        "in": { "name": "$$t.name" }
                    } 
                } 
            }
        }}
    ]
)