Push and slice multiple times in MongoDB - mongodb

Question

I'm trying to do a push a new value to a capped array:
db.messages.insert({name:"test1"})
db.messages.update({name:"test1"}, {"$push":{"output": {"$each": ["test1"], "$slice": -10}}})
db.messages.update({name:"test1"}, {"$push":{"output": {"$each": ["test2"], "$slice": -10}}})
So, the first time I execute the update, I get what I expect:
{
"_id" : ObjectId("51d482ee7252cb3f7eb81ac1"),
"name" : "test1",
"output" : [
"test1"
]
}
But, after the second update, I get the following:
{
"_id" : ObjectId("51d482ee7252cb3f7eb81ac1"),
"name" : "test1",
"output" : [
"test1",
{
"$each" : [
"test2"
],
"$slice" : -10
}
]
}
When I expected the following:
{
"_id" : ObjectId("51d482ee7252cb3f7eb81ac1"),
"name" : "test1",
"output" : [
"test1",
"test2"
]
}
Probably I'm not understanding how to use $push with $slice, but looking at the documentation I couldn't figure out what I'm doing wrong. How can achive adding a new element to a capped array?

Answer 1

The behavior you observed can be reproduced in MongoDB version 2.2. Your example works as expected in MongoDB version 2.4
> db.things.insert({name:"test1"})
> db.things.update({name:"test1"}, {"$push":{"output": {"$each": ["test1"], "$slice": -10}}})
> db.things.find()
{ "_id" : ObjectId("51d489f730d17c1ffbd6ff9f"), "name" : "test1", "output" : [ "test1" ] }
> db.things.update({name:"test1"}, {"$push":{"output": {"$each": ["test2"], "$slice": -10}}})
> db.things.find()
{ "_id" : ObjectId("51d489f730d17c1ffbd6ff9f"), "name" : "test1", "output" : [ "test1", "test2" ] }
From the documentation, $each support to $push operator was added in version 2.4
http://docs.mongodb.org/manual/reference/operator/each/#op._S_each
Most likely you are using an older version of MongoDB that does not support $each with the $push operator. What version of MongoDB are you running? You can find out the version you are running by using the following command from MongoDB shell.
>db.version()
To get the latest release of MongoDB, please go to the following link.
http://www.mongodb.org/downloads