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May 2, 2009
2 thanks

Re: Find random record

Ordering by RAND() is not a wise idea when you have a large table with lots of rows. Your database will have to calculate a different random value for every row in your database – O(N) – then sort the entire table by those values – O(N log N).

There are a number of better ways to get a random record from your table. Some examples:

  • If your table is not sparse, choose a random ID and get that row (or the nearest row):

rand_id = rand(Model.count)
rand_record = Model.first(:conditions => [ "id >= ?", rand_id]) # don't use OFFSET on MySQL; it's very slow
  • If your table is sparse, or does not have a primary key, consider adding an indexed column of random numbers between 0 and N. You can then order by this column quickly and choose a value using a method similar to the above example.

April 30, 2009
1 thank

Find random record

It’s as simple as:

Things.first(:order => 'RAND()')

Of course depending on your database it could be ‘RANDOM()’ or something similar.

April 30, 2009
1 thank

Caveat and design hints regarding :counter_cache

(From Obie Fernandez/ The Rails Way, ISBN 978-0321445612. Thanks Obie!)

This caveat:

The value of the counter cache column must be set to zero by default in the database! Otherwise the counter caching won’t work at all. It’s because the way that Rails implements the counter caching behavior is by adding a simple callback that goes directly to the database with an UPDATE command and increments the value of the counter.

And these tips:

If a significant percentage of your association collections will be empty at any given moment, you can optimize performance at the cost of some extra database storage by using counter caches liberally. The reason is that when the counter cache attribute is at zero, Rails won’t even try to query the database for the associated records!

If you’re not careful, and neglect to set a default value of 0 for the counter cache column on the database, or misspell the column name, the counter cache will still seem to work! There is a magic method on all classes with has_many associations called collection_count, just like the counter cache. It will return a correct count value if you don’t have a counter cache option set or the counter cache column value is null!

April 30, 2009
1 thank

For specific entries, use Dir.glob

When working with the contents of a directory it’s not uncommon to be interested in a specific subset of the entries present.

Dir.glob can be used to fetch entries by name and File.stat can be used to determine the type of file.

April 30, 2009
0 thanks

attachments and implicit multipart

There is a small gotcha - this caught me up for a while.

If you are using implicit multipart mime types by naming your template xxx.text.html.erb and xxx.text.plain.erb, you will need to change your template name back to the original xxx.erb.

If you use the implicit template name, your attachment will be the only thing in the body of the message - it will ignore your template.

See the “Multipart email” section of the ActionMailer.base documentation.

April 30, 2009
0 thanks

Video tutorial

If you want to get up to speed with Rails’ caching and haven’t seen it already, definitely check out this video series on Scaling Rails:

http://railslab.newrelic.com/scaling-rails

April 29, 2009 - (<= v2.3.2)
1 thank

Including instance methods to JSON output

Use :methods parameter to include ActiveRecord instance methods to JSON output. :only and :except uses DB columns only.

@events.to_json(:include => { 
                  :images => { 
                    :only => [], :methods => [:public_url] }})

In the previous example events have multiple images and only public_url instance method is included in the JSON output.

April 28, 2009
1 thank

Moved to ActiveSupport::Inflector

This isn’t gone, it’s just been moved to the ActiveSupport module namespace.

See: ActiveSupport::Inflector#pluralize

April 28, 2009
3 thanks

Tip: Define from_param(...) as Opposite

Often when defining a to_param method, it’s handy to introduce an opposite method for decoding them. For example:

class User < ActiveRecord::Base
  def self.from_param(param)
    find_by_name!(param)
  end

  def to_param
    name
  end
end

While you can just as easily redefine the find() method, this may be confusing since the expectation is that find() works with numerical IDs, or whatever the key column is defined as.

April 28, 2009 - (>= v2.3.2)
4 thanks

A very thorough explanation of use

Ryan Daigle has a great article about 2.3’s new nest forms which does a really good job of explaining how to use this and some of the potential gotchas. Highly recommended:

http://ryandaigle.com/articles/2009/2/1/what-s-new-in-edge-rails-nested-attributes

April 27, 2009 - (>= v2.0.0)
0 thanks

has_one through belongs_to not working

code example:

class Company < ActiveRecord::Base
  has_many :route_lists
end

class RouteList < ActiveRecord::Base
  belongs_to :company
  has_many :routes
end

class Route < ActiveRecord::Base
  belongs_to :route_list
  has_one :company :through => :route_list
end

This creates an invalid SQL query, where the keys in the join between route and routelist are switched, when used as an include:

Routes.find :all, :conditions => ["companies.type = ?", "Account"], :include => :company

route_lists.route_list_id = route.id

instead of: route_lists.id = route.route_list_id

April 25, 2009
4 thanks

Set :use_route to nil to let Rails pick the best route

Imagine the following case. You have two landing pages, one generic one, and an account specific one. The urls are as follows:

map.landing 'landing', :controller => 'landing', :action => 'index'
map.account_landing 'accounts/:account_id/landing', :controller => 'landing', :action => 'index'

Now imagine you want a path to the landing page, using the most specific route possible. If you have an account_id, use it, if not, skip it.

You could do

url_for(:controller => 'landing', :action => 'index', :account_id => current_account)

If current_account is set you’ll get “/accounts/:account_id/landing” if not, you’ll get “/landing”. However, that just looks ugly.

Enter :use_route => nil.

landing_path(:account_id => nil)                    # => '/landing'
landing_path(:account_id => 1)                      # => '/landing?account_id=1'
landing_path(:account_id => nil, :use_route => nil) # => '/landing'
landing_path(:account_id => 1, :use_route => nil)   # => '/accounts/1/landing'

Setting :use_route to nil, is equivalent to the earlier #url_for example.

April 24, 2009
2 thanks

have your to_param begin with the object's id

If you overwrite the to_param method in your model class such that it does not begin with its id, you can be in for a nasty surprise:

Example

class User
  def to_param
    self.login
  end
  ...
end

Let’s say you have a user called “bob”, than you might think this works:

>> bob = User.find(3)
=> #<User id: 3, login: "bob", ...>
>> User.find(bob.to_param)
ActiveRecord::RecordNotFound: Couldn't find User with ID=bob

But it’s not the reason being that Rails find method looks for a beginning number (d+) and uses that to look up the record (and ignores everything that comes after the last digit). So the solution is to have your to_param return something that begins with the object’s id, like so:

Example

class User
  def to_param
    "#{self.id}-#{self.login}"
  end
  ...
end

>> bob = User.find(3)
=> #<User id: 3, login: "bob", ...>

>> User.find(bob.to_param)

> # id: 3, login: “bob”, …>

>> bob.to_param

> “3-bob”

April 24, 2009
0 thanks

test

I need that program :)

April 23, 2009
5 thanks
April 23, 2009
6 thanks

Argument Ordering

Be aware that the order of arguments for this method is the opposite of File.join:

File.expand_path('foo', '/bar')   # => "/bar/foo"
File.join('foo', '/bar')          # => "foo/bar"
April 23, 2009
0 thanks

See Also: IO Class Methods

There are other more specific methods defined in the IO class: IO.open for files, IO.popen for pipes.

April 23, 2009
1 thank

Customize Formatting with a Subclass

Instead of passing in a formatter block, you can always create a subclass that defines the format:

require 'logger'

class MyLogger < Logger
  def format_message(severity, datetime, progname, msg)
    "[%s %s] %s\n" % [ severity, datetime.strtftime("%H:%M"), msg ]
  end
end

This can be easier than always passing the same formatter option.

April 23, 2009
2 thanks

Rails and Ruby 1.8.7 Extensions

Note that the use of Symbol#to_proc requires either Rails or Ruby 1.8.7. Prior versions will show:

['a', 'b', 'c'].collect(&:capitalize)
 #  => TypeError: wrong argument type Symbol (expected Proc)
April 23, 2009
5 thanks

Handy shorthand for array manipulation

You may write something like this:

>> ['a', 'b', 'c'].collect{|letter| letter.capitalize}
=> ["A", "B", "C"]

But it looks so much nicer this way:

>> ['a', 'b', 'c'].collect(&:capitalize)
=> ["A", "B", "C"]
April 23, 2009
1 thank

Using strings as association names

Beware, that using strings as association names, when giving Hash to :include will render errors:

The error occurred while evaluating nil.name

So, :include => [‘assoc1’, ‘assoc2’ ] will work, and :include => [ {‘assoc1’ => ‘assoc3’}, ‘assoc2’] won’t. Use symbols:

Proper form

:include => [ {:assoc1 => :assoc3}, ‘assoc2’]

April 23, 2009 - (v2.0.0 - v2.3.2)
0 thanks

Real HTML_ESCAPE VALUE

Real value:

HTML_ESCAPE = { '&' => '&',  '>' => '>',   '<' => '<', '"' => '"' }
April 21, 2009
3 thanks

Format not coming out properly?

Date, Time and DateTime may have different formats defined.

If you do:

@user.created_at.to_formatted_s(:long_ordinal)

You will get (or something):

April 16th, 2009 22:03 

But if you do:

@user.created_at.to_date.to_formatted_s(:long_ordinal)

You will get:

April 16th, 2009

So, be sure you know which one you are working with.

April 21, 2009
3 thanks

To throw an exception, use Kernel#raise

Other languages use the term throw for raising exceptions, but Ruby has a specific raise call for that.

April 21, 2009
7 thanks

Do not forget to add indexes

Don’t forget to add indexes to HATM table:

add_index :developers_projects, [:developer_id, :project_id]
April 21, 2009
1 thank

Merges with inherited values from super class

http://www.spacevatican.org/2008/8/19/fun-with-class-variables

“When you set a class_inheritable_array or a class_inheritable_hash you are actually concatenating (or merging) with the value inherited from the super class.”

Code example

class Base
  class_inheritable_hash :attrs
  self.attrs = {:name => 'Fred'}
end

class Derived < Base
  self.attrs = {:export => 'Pain'}
end

Derived.attrs #=> {:name => 'Fred', :export => 'Pain'}
April 21, 2009 - (>= v2.3.2)
5 thanks

strip_tags method not functioning in controllers, models, or libs

It comes up with an error about white_list_sanitizer undefined in the class you’re using it in. To get around this, use:

ActionController::Base.helpers.strip_tags('string')

To shorten this, add something like this in an initializer:

class String
  def strip_tags
    ActionController::Base.helpers.strip_tags(self)
  end
end

then call it with:

'string'.strip_tags
April 21, 2009 - (>= v2.3.2)
3 thanks

sanitize method not functioning in controllers, models, or libs

It comes up with an error about white_list_sanitizer undefined in the class you’re using it in. To get around this, use:

ActionController::Base.helpers.sanitize('string')

To shorten this, add something like this in an initializer:

class String
  def sanitize
    ActionController::Base.helpers.sanitize(self)
  end
end

then call it with:

'string'.sanitize