Use mongoid to count array size with aggregate - ruby-on-rails

Question

I'm trying to translate aggregation from the MongoDB shell to ruby code that uses Mongoid as ODM.
I have some documents like this (very simplified example):
{
"name": "Foo",
"tags": ["tag1", "tag2", "tagN"]
},
{
"name": "Bar",
"tags": ["tagA", "tag2"]
},
...
Now I'd like to get all documents with the name field and the total number of tags for each.
In the MongoDB shell I can achieve it using aggregation framework like this:
db.documents.aggregate(
{$project: {name: 1, tags_count: {$size: $tags}}
)
And it will return:
[{"name": "Foo", "tags_count": 3},
{"name": "Bar", "tags_count": 2}]
Now the frustrating part, I'm trying to implement the same query inside a rails app using Mongoid as ODM.
The code looks like (using rails console):
Document.collection.aggregate(
[
{'$project': {name: 1, tags_count: {'$size': '$tags'}}}
]
).to_a
And it returns the next error:
Mongo::Error::OperationFailure: The argument to $size must be an Array, but was of type: EOO (17124)
My question is: How can I make Mongoid understand that $tags makes reference to the correct field? Or what I'm missing from the code?
Thanks

Answer 1

It looks like there is data which does not consistently have an array in the field. For this you can use $ifNull to place an empty array where none is found and thus return the $size as 0:
Document.collection.aggregate(
[
{'$project': {name: 1, tags_count: {'$size': { '$ifNull': [ '$tags', [] ] } } }}
]
).to_a
Alternately you could simply skip where the field is not present at all using $exists:
Document.collection.aggregate(
[
{'$match': { 'tags_count': { '$exists': true } } },
{'$project': {name: 1, tags_count: {'$size': '$tags'}}}
]
).to_a
But of course that will filter those documents from the selection, which may or may not be the desired effect.