### Understanding Ohm Internals through Logging

#### Benefits

# Ohm is actually a very thin wrapper for Redis, and most of the design
# patterns implemented in Ohm are based on the prescribed patterns for using
# Redis.
#
# If you take a little time to grok the internals of Ohm and Redis, the more
# effective you will be in weilding it efficiently.

#### Keys
#
# Most key-value stores have standardized on a structured design pattern for
# organizing data. Let's fire up a console:
#
#     >> irb -r ohm -r logger
#
#     Ohm.redis.client.logger = Logger.new(STDOUT)
#
#     class Post < Ohm::Model
#       attribute :title
#     end
#
#     Post.create(:title => "Grokking Ohm")
#
# After executing `Post.create(...)`, you'll see a lot of output from the
# logger. Let's go through every line in detail.

#### Post.create

# *Generate a new `Post:id`.*
INCR Post:id

# *Lock `Post:2`
# (see [SETNX](http://code.google.com/p/redis/wiki/SetnxCommand)
# for an explanation of the locking design pattern).
SETNX Post:2:_lock 1285060009.409451

# *Add the newly generated ID 2 to the `Post:all` SET.*
SADD Post:all 2

# *Start transaction.*
MULTI

# *Delete HASH `Post:2`.*
DEL Post:2

# *Set the attributes.*
HMSET Post:2 title "Grokking Ohm"

# *End transaction.*
EXEC

# *Release the lock.*
DEL Post:2:_lock

#### Sets and Lists

# Let's do a little `SET` and `LIST` manipulation and see what Ohm does.
# Here's the code for this section:
#
#     class User < Ohm::Model
#       collection :posts, Post
#     end
#
#     class Post < Ohm::Model
#       attribute :title
#       reference :user, User
#
#       list :comments, Comment
#     end
#
#     class Comment < Ohm::Model
#     end

#### User.create

# *Generate a new ID for the `User`.*
INCR User:id

# *Lock User:9.*
SETNX User:9:_lock 1285061032.174834

# *Add this new user to the SET User:all.*
SADD User:all 9

# *Release the lock.*
DEL User:9:_lock

#### Post.create(:title => "Foo", :user => User.create)

# *Generate an ID for this Post*
INCR Post:id

# *Lock Post:3*
SETNX Post:3:_lock 1285061032.180314

# *Add the ID 3 to the SET Post:all*
SADD Post:all 3

# *Start transaction*
MULTI

# *Remove existing Post:3 HASH*
DEL Post:3

# *Assign the attributes to Post:3*
HMSET Post:3 title Foo user_id 9

# *End transaction*
EXEC

# *Add the ID of this post to the SET index (more on this below)*
SADD Post:user_id:OQ== 3

# *Book keeping for Post:3 indices*
SADD Post:3:_indices Post:user_id:OQ==

# *Release lock*
DEL Post:3:_lock

#### post.comments << Comment.create

# *Generate an ID for this comment*
INCR Comment:id

# *Lock Comment:1*
SETNX Comment:1:_lock 1285061034.335855

# *Add this comment to the SET Comment:all*
SADD Comment:all 1

# *Release lock*
DEL Comment:1:_lock

# *Append the comment to the LIST Post:3:comments*
RPUSH Post:3:comments 1

#### Understanding indexes

#     reference :user, User
#
# is actually more or less the same as:
#
#     attribute :user_id
#     index :user_id
#
#     def user
#       User[user_id]
#     end
#
#     def user_id=(user_id)
#       self.user_id = user_id
#     end
#
# To further explain the [example above](#section-29), let's
# run through the commands that was issued. Remember that
# we had a *user_id* of *9* and a *post_id* of *3*.

# *OQ== is Base64 of *9*, with newlines removed. The post_id 3 is added
# effectively to Post:user_id:9, if we ignore the encoding.*
SADD Post:user_id:OQ== 3

# *Just keep track that Post:user_id:OQ== was created*
SADD Post:3:_indices Post:user_id:OQ==

# *Get all *posts* with user_id *9**
SMEMBERS Post:user_id:OQ==

#### What's next?

# We tackled a fair bit amount regarding Ohm internals. More or less this is
# the foundation of everything else found in Ohm, and other code are just
# built around the same fundamental concepts.
#
# If there's anything else you want to know, you can hang out on #ohm at
# irc.freenode.net, or you can visit the
# [Ohm google group](http://groups.google.com/group/ohm-ruby).