How to pass values to named scope?

Rails
1

In my Video model i have:

class Video < Media
scope :genre, where([‘genres.name = ?’,
params[:genre].capitalize]).includes({:artist => {:user => :genre}})
…
end

As you see above, params[:genre].capitalize is a dynamic value. How do
I pass a value when I call Video.genre?

Or are we not allowed to pass attributes/values to named scopes?

Should I use a function instead? For example:

def find_by_genre(genre)
self.where([‘genres.name = ?’, genre).includes({:artist => {:user
=> :genre}})
end

2

Christian F. wrote:

In my Video model i have:

class Video < Media
scope :genre, where([‘genres.name = ?’,
params[:genre].capitalize]).includes({:artist => {:user => :genre}})
…
end

As you see above, params[:genre].capitalize is a dynamic value. How do
I pass a value when I call Video.genre?

Or are we not allowed to pass attributes/values to named scopes?

Read the AR rdoc on named_scopes. It explains this quite clearly.
(Hint: lambdas are involved.)

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

3

If thats the case, when should one use a named scope over a method?

For example:

Product.find_by_category(category).

find_by_category can either be a named scope or a method in the
Product model. I was under the impression that named scopes cant
accept an attribute and this was the main difference. Apparently, it
can!

So when does one use named scopes over methods?

4

On Oct 8, 9:30 pm, Christian F. [email protected]
wrote:

So when does one use named scopes over methods?

(Nitpicking: a call to named_scope (or scope in rails 3) does just
define a method of a particular form)
Scopes are chainable which can be very handy. On the other hand there
is only so much a scope can do - scopes boil down to a single query to
the database, so if you need to do more, then you’ll probably want to
wrap up the extra work in a method.

Fred

5

Sometimes if I need to do a calculation before I call a named scope or
if I want to make a different query based on the input, I do something
like this:

class Media < AR:Base
def dvds(name = “”)
if name.blank?
Media.scoped({})
else
name.downcase!
Media.scoped(:conditions => {:name => name})
end
end
end

In either case, it’s still chainable. This is Rails2 code, but I’m sure
you could do something similar with Rails3.

Luke

6

On 9 October 2010 00:44, Luke C. [email protected] wrote:

end
end

Could that just be
Media.scoped( :conditions => {:name => name.downcase!} unless
name.blank? )
or something similar, I have not tried it.

Colin

7

Please quote when replying.

Christian F. wrote:

If thats the case, when should one use a named scope over a method?

For example:

Product.find_by_category(category).

find_by_category can either be a named scope or a method in the
Product model. I was under the impression that named scopes cant
accept an attribute and this was the main difference. Apparently, it
can!

So when does one use named scopes over methods?

A named_scope is a method, or as near as matters. Use whichever is
easier to implement and otherwise more suitable to your use case.

Best,

Marnen Laibow-Koser
http://www.marnen.org
[email protected]

8

Hi Colin, yes something like what you’ve proposed would work. This is
the most concise syntax I could come up with to get your suggestion
working.

Media.scoped(:conditions => (name.blank? ? {:name => name.downcase!} :
{}))

My suggestion (below) was to illustrate that you could have something
that looks like a named scope (dvds) actually be a method. At some point
the logic you need to apply may not fit nicely into a single named_scope
call and I find this to be a good way to handle these situations.

Luke