MongoDB Aggregation Framework: Getting $unwind error when using $group - mongodb

Question

I have a document structure as follows:
{
"_id" : NumberLong("80000000012"),
[...]
"categories" : [{
"parent" : "MANUFACTURER",
"category" : "Chevrolet"
}, {
"parent" : "MISCELLANEOUS",
"category" : "Miscellaneous"
}],
[...]
}
I am trying to get a distinct list of all 'category' fields for each 'parent' field. I was trying to utilize the aggregation framework to do this with the following query:
db.posts_temp.aggregate(
{$unwind : '$categories'},
{$match : {'categories.parent' : 'MISCELLANEOUS'}},
{$project : {
'_id' : 0,
parent : '$categories.parent',
category : '$categories.category'
}
},
{
$group : {
_id : '$parent',
category : {$addToSet : '$category'}
}
}
);
Running this query returns the following error:
{
"errmsg" : "exception: $unwind: value at end of field path must be an array",
"code" : 15978,
"ok" : 0
}
This seems to be tied to the group portion of the query, because, when I remove it, the query runs correctly, but, obviously, the data is not where I want it to be.

Answer 1

I just tried executing the above aggregation query on my mongo instance. Here are my 3 documents each with a key of categories that has an array of two nested documents.
Here is my data:
{
"_id" : ObjectId("512d5252b748191fefbd4698"),
"categories" : [
{
"parent" : "MANUFACTURER",
"category" : "Chevrolet"
},
{
"parent" : "MISCELLANEOUS",
"category" : "Miscellaneous"
}
]
}
{
"_id" : ObjectId("512d535cb748191fefbd4699"),
"categories" : [
{
"parent" : "MANUFACTURER",
"category" : "Chevrolet"
},
{
"parent" : "MISCELLANEOUS",
"category" : "Pickup"
}
]
}
{
"_id" : ObjectId("512d536eb748191fefbd469a"),
"categories" : [
{
"parent" : "MANUFACTURER",
"category" : "Toyota"
},
{
"parent" : "MISCELLANEOUS",
"category" : "Miscellaneous"
}
]
}
Here is the aggregation query of yours that I ran:
db.posts_temp.aggregate( {$unwind:'$categories'} , {$match: {'categories.parent':'MISCELLANEOUS'}}, {$project:{'_id':0, parent: '$categories.parent', category:'$categories.category'}}, {$group:{_id:'$parent', category:{$addToSet:'$category'}}})
Here is the result:
{
"result" : [
{
"_id" : "MISCELLANEOUS",
"category" : [
"Pickup",
"Miscellaneous"
]
}
],
"ok" : 1
}
Let me know if there some discrepancies between my data and yours.
CSharpie