Is there a way to prevent mongo queries "branching" on arrays?

Question

If I have the following documents:

{a: {x:1}} // without array
{a: [{x:1}]} // with array

Is there a way to query for {'a.x':1} that will return the first one but not the second one? IE, I want the document where a.x is 1, and a is not an array.

Answer 1

Please note that future version of MongoDB would incorporate the $isArray aggregation expression. In the meantime...

...the following code will do the trick as the $elemMatch operator matches only documents having an array field:

> db.test.find({"a.x": 1, "a": {$not: {$elemMatch: {x:1}}}})

---

Given that dataset:

> db.test.find({},{_id:0})
{ "a" : { "x" : 1 } }
{ "a" : [ { "x" : 1 } ] }
{ "a" : [ { "x" : 0 }, { "x" : 1 } ] 

It will return:

> db.test.find({"a.x": 1, "a": {$not: {$elemMatch: {x:1}}}}, {_id:0})
{ "a" : { "x" : 1 } }

---

Please note this should be considered as a short term solution. The MongoDB team took great cares to ensure that [{x:1}] and {x:1} behave the same (see dot-notation or $type for arrays). So you should consider that at some point in the future, $elemMatch might be updated (see JIRA issue SERVER-6050). In the meantime, maybe worth considering fixing your data model so it would no longer be necessary to distinguish between an array containing one subdocument and a bare subdocument.