This is a really confusing topic. I work at 10gen and I had to spend a while wrapping my head around it 😉
Let’s walk through how the query engine processes this query.
Here’s the query again:
> db.test.find({ b : { $gt : 4, $lt : 6}});
When it gets to the record that seems like it shouldn’t match…
{ "_id" : ObjectId("4d54cff54364000000004331"), "a" : 1, "b" : [ 2, 4, 6, 8 ] }
The match is not performed against each element of the array, but rather against the array as a whole.
The comparison is performed in three steps:
Step 1: Find all documents where b has a value greater than 4
b: [2,4,6,8] matches because 6 & 8 are greater than 4
Step 2: Find all documents where b has a value less than 6
b: [2,4,6,8] matches because 2 & 4 are less than 6
Step 3: Find the set of documents that matched in both step 1 & 2.
The document with b: [2,4,6,8] matched both steps 1 & 2 so it is returned as a match. Note that results are also de-duplicated in this step, so the same document won’t be returned twice.
If you want your query to apply to the individual elements of the array, rather than the array as a whole, you can use the $elemMatch operator. For example
> db.temp.find({b: {$elemMatch: {$gt: 4, $lt: 5}}})
> db.temp.find({b: {$elemMatch: {$gte: 4, $lt: 5}}})
{ "_id" : ObjectId("4d558b6f4f0b1e2141b66660"), "b" : [ 2, 3, 4, 5, 6 ] }