indexes on sub-collections

[samullen]

I'm just curious how I would go about indexing "sub-collections" (not
sure what to call those).

Example:
I have a user collection. Users can have followers and be followers
(following) of other users.

class User
include MongoMapper::Document

<user fields>

many :followers
many :followings
end

class Follower
include MongoMapper::Document

key :follower_id, String, :required => true
key :blocked, Boolean, :default => false
timestamps!
end

class Following
include MongoMapper::Document

key :following_id, String, :required => true
timestamps!
end

So how do I index the followers/followings? Do I put them in the
respective classes
MongoMapper.ensure_index(Follower, :following_id, :unique => true)
MongoMapper.ensure_indexes!

or do I put the indexes in the User table? If I do this, is it the
same syntax as previous?

Thanks,
Samuel

[John Nunemaker]

In the respective classes. All the many does is create associations
that do querying for you behind the scenes.

Also, don't use MongoMapper.ensure_index directly. Just put your
indexes in your model. For example, in the following model you could
do this:

key :following_id, String, :required => true, :index => true

or you could do this...

ensure_index :following_id

Also, this gist shows an initializer to use database.yml to store your
database connection settings and ensure that all indexes are created
at boot time in rails.

http://gist.github.com/232953

Hope that helps.

[Kyle Banker]

If you're going to model your data in this way, then you'll want to create indexes in both the Follower and Following models on each of the foreign keys.

That said, I think there's a more document-oriented way of modeling this data.  Why not express followers, following, and blocked in the user model itself?

class User
 include MongoMapper::Document

 <user fields>

  key :followers,  Array
  key :following,  Array
  key :blocked,   Array
end

Each of the fields, followers, following, and block, will contain a user id.  If you place a unique index on each of those fields, then queries for all of a user's followers, or for everyone a user is following, can still be efficient.

Find all of Bob's followers:
User.all(:conditions => {:following => @bob.id}

Find everyone Bob is following:
User.all(:conditions => {:following => @bob.id}

Of course, writes are a little more complex, but this is likely a worthwhile tradeoff.

[John Nunemaker]

I agree with Kyle and should have stated this. I would use array of ids instead of full models and just index those arrays. We are doing this *a lot* in Harmony an app we are building.

[samullen]

Outstanding! Works just like I thought it would. And thanks for the
gist.

Samuel

[samullen]

I chose to do full Models because 1) it's not just users which can be
followed; and 2) so I could use model specific methods of said
models.

I may have to rethink things a little more.

Thanks again,
Samuel

[Cyril Mougel]

samullen a écrit :

> I chose to do full Models because 1) it's not just users which can be
> followed; and 2) so I could use model specific methods of said
> models.
>
> I may have to rethink things a little more.
>

You can have a embeddedDocument too. With embedded you can have a lot of
data. After with an adequate callback you can update it.

--
Cyril Mougel
http://blog.shingara.fr/

[samullen]

Yeah, I didn't quite realize how "many" worked. I thought it built out
the embedded document. I'm going to switch over to using hashes for my
followers and followings. I can bring in blocked at that point.

Samuel

[John Nunemaker]

many works with embedded documents or documents. If embedded then it
embeds, if document then it does foreign key type queries.

[samullen]

John,

Thanks. I just saw that I was supposed to be using
"MongoMapper::EmbeddedDocument" and not just "MongoMapper::Document".

Hate to be such a nube.

Samuel

[Aaron]

I notice when using many the foreign key shouldn't be a string because
the foreign key will be wrapped in quotes.

class User
include MongoMapper::Document

many :posts
end

User.first.posts << Post.new()

User.first.posts # => MONGODB app-development.posts.find
({:user_id=>4b04c9ba9a67c268aa000001}, {})
Notice the key isn't wrapped in quotes.

Then we add the foreign key in Post

class Post
include MongoMapper::Document

key :user_id, String, :required => true, :index => true
end

Post.first # => #<Post user_id: "4b04c9ba9a67c268aa000001">
Notice the key IS wrapped in quotes.
User.first.posts # => [ ]


class Post
include MongoMapper::Document

key :user_id, :required => true, :index => true
end

Pull out the String from the key.
Post.first # => #<Post user_id: 4b04c9ba9a67c268aa000001>
User.first.posts # => [ #<Post user_id: "4b04c9ba9a67c268aa000001"> ]

Maybe I'm missing something here but it works now. Thanks to the
logger I noticed 'many' searches for foreign keys without quotes.

[John Nunemaker]

String is only an option for id's and foreign keys for backwards
compatibility. From now on every foreign key should be ObjectId type.

key :user_id, ObjectId, :required => true, :index => true

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[Aaron]

Sorry about that, I was going through a newer post and caught the
ObjectId part right after I posted.

On Nov 23, 4:34 am, John Nunemaker <nunema...@gmail.com> wrote:
> String is only an option for id's and foreign keys for backwards
> compatibility. From now on every foreign key should be ObjectId type.
>
> key :user_id, ObjectId, :required => true, :index => true
>