MongoDB: Find documents with key in subdocument - mongodb

Question

The $in operator works with arrays.
Is there an equivalent for dictionaries?
The following code creates two test documents and finds the ones containing one of the listed values in the array documents, but doesn't find the ones containing the same values in the sub-documents.
> use test
> db.stuff.drop()
> db.stuff.insertMany([{lst:['a','b'],dic:{a:1,b:2}},{lst:['a','c'],dic:{a:3,c:4}}])
{
"acknowledged" : true,
"insertedIds" : [
ObjectId("595bbe8b3b0518bcca4b1530"),
ObjectId("595bbe8b3b0518bcca4b1531")
]
}
> db.stuff.find({lst:{$in:['b','c']}},{_id:0})
{ "lst" : [ "a", "b" ], "dic" : { "a" : 1, "b" : 2 } }
{ "lst" : [ "a", "c" ], "dic" : { "a" : 3, "c" : 4 } }
> db.stuff.find({dic:{$in:['b','c']}},{_id:0})
>
EDIT (in response to the answer below)
Using the list as suggested in the answer below prevents me from finding the desired element. For example, after executing both the insertMany above in this question and below in the answer, the following can be done with a dictionary, not with a list (or am I missing something?):
> x=db.stuff.findOne({lst:{$in:['b','c']}},{_id:0})
{ "lst" : [ "a", "b" ], "dic" : { "a" : 1, "b" : 2 } }
> x
{ "lst" : [ "a", "b" ], "dic" : { "a" : 1, "b" : 2 } }
> x.dic.a
1
> x.dic.b
2

Answer 1

For subdocuments, there's no exact equivalent to $in. You could use the $exists query operator combined with $or:
db.stuff.find({$or:[
{'dic.b': {$exists: true}},
{'dic.c': {$exists: true}}
]})
The recommended approach, however, is to change your schema, so that the keys and values are changed into an array of {key: "key", value: 123} subdocuments:
db.stuff.insertMany([
{dic: [{key: 'a', value: 1}, {key: 'b', value: 2}]},
{dic: [{key: 'a', value: 3}, {key: 'c', value: 4}]}
])
Then you can use $in to find documents with certain keys:
db.stuff.find({'dic.key': {$in: ['a', 'b']}})
The especially good thing about this new schema is you can use an index for the $in query:
db.stuff.createIndex({'dic.key': 1})
A disadvantage, as you point out above, is that simple element access like x.dic.a no longer works. You need to do a bit of coding in your language. E.g. in Javascript:
> var doc = {dic: [{key: 'a', value: 3}, {key: 'c', value: 4}]}
> function getValue(doc, key) {
... return doc.dic.filter(function(elem) {
... return elem.key == key;
... })[0].value;
... }
> getValue(doc, "a")
3
> getValue(doc, "c")
4