How to pass values to named scope?

rubyonrails-talk
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 Fazzini 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,

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

Please quote when replying.

Christian Fazzini 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,

5

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?

(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

6

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

7

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

Colin

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