After updating a few apps to Rails 3 I came across some issues with the new scoping syntax.
Ambiguous column names
In one case I use a module to define some common scopes that are used across more than one model. I quickly came up against the ambiguous column name error (e.g. Column ‘created_at’ in order clause is ambiguous —from MySQL). To fix this I used the table_name prefix right in my scoping string. In general you should always do this anywhere you use strings in your scopes. For example;
# remember to add the table_name to avoid ambiguous columns
scope :recent, order("#{table_name}.created_at DESC")
# or directly
scope :recent, order("questions.created_at DESC")
# you don't have to add the table_name when using a hash, Rails takes care of that!
scope :by_user, lambda{|user| where(:user_id => user.id).recent}
Multiple order scopes
Another issue I found was combining multiple order scopes. In Rails 2 with named scopes this used to work as you’d expect. But combining multiple order scopes in Rails 3 seems to fail, only applying the last order scope to the query.
# given the recent scope defined above, in this case we'd expect;
scope :flagged, order(" #{table_name}.flag_count DESC").recent
# to be
'ORDER BY questions.flag_count DESC, questions.created_at DESC'
# BUT its not! instead its simply
'ORDER BY questions.created_at DESC'
# to order on multiple columns inside a scope, I have to duplicate my recent order scope like so
scope :flagged, order(" #{table_name}.flag_count DESC, #{table_name}.created_at DESC").recent
All in all, i’m very much in favour of the new syntax and AREL in general. It simplifies compounding queries together with additional scopes and delays their execution until it is needed.
