MongoDB distinct query against an array of objects - mongodb

Question

If I have a schema similar to the following:
{
"_id" : "12345",
"_class" : "Refrigerator",
"items" : {
"APPLE" : {
"id" : 123,
"name" : "APPLE"
},
"BANANA" : {
"id" : 234,
"name" : "BANANA"
},
"STRAWBERRY" : {
"id" : 345,
"name" : "STRAWBERRY"
}
},
},
{
"_id" : "23456",
"_class" : "Pantry",
"items" : {
"CEREAL" : {
"id" : 456,
"name" : "CEREAL"
}
},
}
I want to get a list of distinct items where the _class=Refrigerator. The result should be ["APPLE","BANANA","STRAWBERRY"].
How can I do this? Thank you!

Answer 1

You can utilise aggregation operator $objectToArray (SERVER-23310) to turn keys into values. It should be able to group 'unknown' or 'dynamic' fields. This operator is available in MongoDB v3.4.4+
Given your example documents, you can use below aggregation pipeline
db.collection.aggregate([
{$match:{"class":"Refrigerator"}},
{$project:{"items":{$objectToArray:"$items"}}},
{$unwind:"$items"},
{$group:{_id:"$_id", result:{$addToSet:"$items.k"}}}
])
See also the manual for: $unwind, $group, and $addToSet
If applicable for your use case, I would recommend to re-consider your data schema for easier access to the information that you're after. MongoDB has a flexible schema, you should use this for your application benefits.