The ELC Community Blog

A knowledge exchange on Ruby on Rails and Agile Development

ActiveRecord attribute update semantics

by Ryan Garver on February 20, 2007

ActiveRecord (AR) provides a number of great features. One feature that is often taken for granted is the mapping of table columns (attributes) to class signals. We use this feature of AR without thinking about it.

  
   1  article = Article.find(1)
   2  article.attributes[:title] #=> "A Blog Posting"
   3  article.title #=> "A Blog Posting"

We also can take this several steps further and modify how AR responds to requests for the value of a attribute. Jamis Buck had a recent blog post outlining an interesting way to set up default values for an attribute than can depend on another attribute. The difficulty in AR comes when you need to modify what happens when attributes are being updated. A great example of this is if you want an action to always happen when (say) you assign an

   1  Author

   1  Article.author

   1  article.attributes[:attr_name]=value

Every AR object has an

   1  attributes

Hash that contains the

   1  name=>value

pairs that correspond to the columns and their values for the object. This Hash is bi-directional and therefore can be modified. Once the

   1  attributes

Hash has been updated you can save the AR object and those updates will persist. Much of the alternative methods of updating AR objects are built on this base. The

   1  attr_name

can be either a string or a symbol.

   1  article.attr_name=value

This assigns

   1  attr_name

with

   1  value

within the in-memory AR object. To commit the change you must call

   1  save

on the AR object. This ends up calling

   1  []=

under the hood. When overwriting how the attributes are updated in a AR object you'll want to override this method and do the final assignment using

   1  []=

.

   1  article.update_attribute(:attr_name, value)

This assigns the

   1  value

to

   1  attr_name

and saves the value to the database all in one go. Behind the scene this calls the

   1  article.attr_name=

to do the assignment which means that you can override how this assignment takes place. This method of assignment also skips validation on the attribute.

   1  article.update_attributes(:attr_name1 => value1, :attr_name2 => value2, ...)

This method is the same as calling

   1  []=

for each

   1  name=>value

pair and then saving.

A quick note. When overloading the assignment of association do not try to do the overloading on (for example) the

   1  author=

   1  author_id=