How to get mongodb deeply embeded document id - mongodb

Question

I have the following mongo document, which is part of a bigger document called attributes, which also has Colour and Size
> db.attributes.find({'name': {'en-UK': 'Fabric'}}).pretty()
{
"_id" : ObjectId("543261cda14c971132fa2b91"),
"values" : [
{
"source" : [
{
"_id" : ObjectId("543261cda14c971132fa2b79"),
"name" : {
"en-UK" : "Combed Cotton"
}
},
],
"name" : [
{
"_id" : ObjectId("543261cda14c971132fa2b85"),
"name" : {
"en-UK" : "Brushed 3-ply"
}
},
{
"_id" : ObjectId("543261cda14c971132fa2b8f"),
"name" : {
"en-UK" : "Plain Weave"
}
},
{
"_id" : ObjectId("543261cda14c971132fa2b90"),
"name" : {
"en-UK" : "1x1 Rib"
}
}
]
}
],
"name" : {
"en-UK" : "Fabric"
}
}
I am trying to return the _id for a sub document and have the following:
db.attributes.aggregate([
{ '$match': {'name.en-UK': 'Fabric'} },
{ '$unwind' : '$values' },
{ '$project': { 'name' : '$values.name'} },
{ '$match': { '$and': [{"name.name.en-UK" : "1x1 Rib"} ] }}
])
What is the correct way to do this?
Also, the values of Fabric is an array with two items, source and name, but if I populate it like:
> db.attributes.find({'name': {'en-UK': 'Fabric'}}).pretty()
{
"_id" : ObjectId("543261cda14c971132fa2b91"),
"values" : {
"source" : [{ ... }]
"name": [{ ... }]
}
}
I get the following error
"errmsg" : "exception: $unwind: value at end of field path must be an array"
But if I wrap it inside a square brackets this then works, so that
> db.attributes.find({'name': {'en-UK': 'Fabric'}}).pretty()
{
"_id" : ObjectId("543261cda14c971132fa2b91"),
"values" : [{
"source" : [{ ... }],
"name": [{ ... }]
}]
}
what am I missing as values is an array of two objects, source and name each containing a list of arrays
Any advice much appreciated

Answer 1

What you seem to be "missing" here is that "some" of your documents do either not contain a "value" property at all or at the very least it is "not an array". This is the basic context of the error you have been given.
Fortunately there are a couple of ways to get around this. Namely, either "testing" for the presence of an array when submitting you original query. Or actually "substituting" the missing element for some kind of array when processing the pipeline.
Here are both approaches in what is effectively an redundant form since the first $match condition really sorts this out:
db.attributes.aggregate([
{ "$match": {
"name.en-UK": "Fabric",
"values.0": { "$exists": true }
}},
{ "$project": {
"name": 1,
"values": { "$ifNull": [ "$values", [] ] }
}},
{ "$unwind": "$values" },
{ "$unwind": "$values.name" },
{ "$match": { "values.name.name.en-UK" : "1x1 Rib" }}
])
So as I said. Really redundant in that the initial $match actually asks if an "initial array element" actually exists. Which kind of means that there is an array there.
The second $project phase actually uses the $ifNull operator to "fill in" a value ( or basically an empty array ) where the tested element does not exist. We tested for that anyway before, but this demonstrates the different approaches.
But the basic idea id either "avoiding" or "filling-in" where your document does not have the expected data that you want to process. Which is the cause of your error.