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 a

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" }
                    } 
                } 
            }
        }}
    ]
)