How to search for child objects inside parent objects in MongoDB? - mongodb

Question

I'm trying to search any value that match with a "name" param, inside any object with any level in a MongoDB collection.
My BSON looks like this:
{
"name": "a",
"sub": {
"name": "b",
"sub": {
"name": "c",
"sub": [{
"name": "d"
},{
"name": "e",
"sub": {
"name": "f"
}
}]
}
}
}
I've created an index with db.collection.createIndex({"name": "text"}); and it seems to work, because it has created more than one.
{
"numIndexesBefore" : 1,
"numIndexesAfter" : 6,
"note" : "all indexes already exist",
"ok" : 1
}
But, when I use this db.collection.find({$text: {$search : "b"}}); to search, it does not work. It just searches at the first level.
I cannot do a search with precision, because the dimensions of the objects/arrays is dynamic and can grow or shrink at any time.
I appreciate your answers.

Answer 1

MongoDB cannot build an index on arbitrarily-nested objects. The index only occurs for the depth specified. In your case, the $text search will only check the top-level name field, but not the name field for any of the nested sub-documents. This is an inherent limitation for indexing.
To my knowledge, MongoDB has no support for handling these kinds of deeply-nested data structures. You really need to break your data out into separate documents in order to handle it correctly. For example, you could break it out into the following:
[
{
"_id": 0,
"name": "a",
"root_id": null,
"parent_id": null
},
{
"_id": 1,
"name": "b",
"root_id": 0,
"parent_id": 0
},
{
"_id": 2,
"name": "c",
"root_id": 0,
"parent_id": 1
},
{
"_id": 3,
"name": "d",
"root_id": 0,
"parent_id": 2
},
{
"_id": 4,
"name": "e",
"root_id": 0,
"parent_id": 2
},
{
"_id": 5,
"name": "f",
"root_id": 0,
"parent_id": 4
}
]
In the above structure, our original query db.collection.find({$text: {$search : "b"}}); will now return the following document:
{
"_id": 1,
"name": "b",
"root_id": 0,
"parent_id": 0
}
From here we can retrieve all related documents by retrieving the root_id value and finding all documents with an _id or root_id matching this value:
db.collection.find({
$or: [
{_id: 0},
{root_id: 0}
]
});
Finding all root-level documents is a simple matter of matching on root_id: null.
The drawback, of course, is that now you need to assemble these documents manually after retrieval by matching a document's parent_id with another document's _id because the hierarchical information has been abstracted away. Using a $graphLookup could help alleviate this somewhat by matching each subdocument with a list of ancestors, but you would still need to determine the nesting order manually.
Regardless of how you choose to structure your documents moving forward, this sort of restructure is going to be needed if you're going to query on arbitrarily-nested content. I would encourage you to consider different possibilities and determine which is most suited for your specific application needs.