For example a tree structure as;
[
{id: 1 , childrenIdList: [2, 3]},
{id: 2 , childrenIdList: [4, 5]},
{id: 3 , childrenIdList: []},
{id: 4 , childrenIdList: [6, 7]},
{id: 5 , childrenIdList: []},
{id: 6 , childrenIdList: []},
{id: 7 , childrenIdList: []}
]
which is like;
1
2 3
4 5
6 7
How can I trace tree from starting the leaf node(id=7) to root(id=1)?
Finding the parent of id=7 is easy as;
db.document.find({childrenList: { $in: [7]}}, {id: 1}).toArray(function(err), result{
/*result gives
{"id" : NumberInt(4)}
now I should look the parent of id=4, and parent of id=2 as you know.
*/
})
Is recursive queries possible on mongodb? How can I implement it?
Depending on your use case, MongoDB v3.4 provides an aggregation pipeline operator called $graphLookup. The aggregation operator is able to perform a recursive search on a collection. See more definiton on $graphLookup definition.
Using your documents hierarchy and values above as examples, you could try running below aggregation:
db.collectionName.aggregate([
{$unwind:{
path:"$childrenIdList",
preserveNullAndEmptyArrays: true}
},
{$graphLookup:{
from:"collectionName",
startWith:"$_id",
connectFromField:"_id",
connectToField:"childrenIdList",
as:"myparents",
restrictSearchWithMatch: {"_id"}}
},
{$match: {"_id": 7 } },
{$group:{
_id:"$_id",
parents:{$addToSet:"$myparents._id"}
}}
]);
The above should return result as below:
{ "_id" : 7, "parents" : [ [ 1, 2, 4 ] ] }
Having said that, if you have a large collection the above query may not be performant as you'll be performing $unwind on each documents and won't be able to utilise indexes. As suggested by others, you should re-consider your document model structure. See Data Models Tree Structures. Optimise based on your application logic and querying use case, and let the flexible document schema follow.