Would properly using an index prevent so many queries?

Question

I'm using MongoDB (with Mongoid) have a query which runs really really often in my app, I'm not sure I have proper indexes set up to make the queries more efficient, here is a snippet from my log when the query runs:

MONGODB (33ms) recipes_development['system.namespaces'].find({})
MONGODB (6ms) recipes_development['recipes'].find({})
MONGODB (0ms) recipes_development['system.namespaces'].find({})
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48469d4064241d860002d9')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48469d4064241d8600032a')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48469a4064241d8600008d')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48469c4064241d86000273')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48469d4064241d86000365')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f4846a34064241d86000910')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48499a4064241c58000027')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48499a4064241c58000027')}).limit(-1).sort([[:created_at, :desc]])
MONGODB (0ms) recipes_development['users'].find({:_id=>BSON::ObjectId('4f48499a4064241c58000027')}).limit(-1).sort([[:created_at, :desc]])
... many more ...

How could I make this more efficient, or is the behavior I'm seeing typical?

Update

So the way I'm getting these records is via these scopes:

scope :active, where(:active => true)
default_scope order_by([:created_at, :desc])

Then I set the instance varibale in my controller thusly:

@users = User.active

What command in Mongoid are you issuing to get all these users? — Jesse Wolgamott, Mar 02 '12 at 03:22
Hey Jesse, I updated my question, I should have included that in the first place huh? Thanks for you help! — JP Silvashy, Mar 02 '12 at 04:52
It looks like a classic n+1 problem... Are you sure that in the view you're not looking over all the recipes? — Jesse Wolgamott, Mar 02 '12 at 16:04
He's looking for users, recipes are not involved. But the query seems strange, there's no mention of `active` in it. — Sergio Tulentsev, Mar 03 '12 at 03:32
Joseph: is this whole output the result of single `User.active` invocation, or several? — Sergio Tulentsev, Mar 03 '12 at 03:33

score 1 · Accepted Answer · answered Mar 03 '12 at 04:27

This appears to be an N+1 problem and you can fix it by using eager loading. An explanation of eager loading in Mongo for mongo mapper can be found here (search for "Eager Loading"):

Would properly using an index prevent so many queries?

1 Answers1