How are class variables persisted using Activerecord?
Cheers,
Ganesh G.
Hey,
Could you elaborate on the question? Perhaps post an example of your
question so we can help you.
Steve
If you are asking about actual class and class instance variables
here’s a brief explanation. If you’re talking about variables that
show up in view templates or somewhere else then please be more
descriptive with your question.
Class variables in ruby persist throughout a whole class hierarchy.
class A
@@a = 1
def self.a ; @@a ; end
end
A.a # => 1
class B < A
@@a = 2
end
A.a # => 2
B.a # => 2
Class instance variables on the other hand exist in the context of a
single class definition.
class C
@c = 1
def self.c ; @c ; end
end
C.c # => 1
class D < C
@c = 2
end
C.c # => 1
D.c # => 2
Zach
On Apr 26, 12:27 pm, Ganesh G. <rails-mailing-l…@andreas-
Maybe dumb question… why not just use count? That’ll be up to date. If
you want to avoid doing a lot of queries, then you can query any time
you don’t have a cached value, and otherwise just increment the cached
value.
Stephen B. wrote:
Hey,
Could you elaborate on the question? Perhaps post an example of your
question so we can help you.Steve
Hmm,
Let us say I have a User class that inherits from ActiveRecord::Base
class User < ActiveRecord::Base
end
The User has two attributes :name and :age
class CreateUsers < ActiveRecord::Migration
def self.up
create_table :users do |t|
t.column :name, :string
end
end
def self.down
drop_table :users
end
end
All the attributes are persisted and are also available thro the
instance variable @attributes.
ex: user = User.create(:name => “gg”, :age => 25)
p user.name #=> “gg”
p user.attributes[“name”] #=> “gg”
So effectively the instance variables are persisted in the model.
Let us say I need to track the number of users created. So basically I
have to maintain the number of users value in the Class level (I
understand that we can also store it in a separated model/table, but
storing it in the class level makes more sense.)
Ignoring error checking “User” model will now look like
class User < ActiveRecord::Base
@@no_of_users = 0
def initialize
@@no_of_users += 1
end
def User.no_of_users
@@no_of_users
end
end
user1 = User.new
p User.no_of_users # => 1
user2 = User.new
p User.no_of_users # => 2
The problem is, we are not persisting the class variable in the
database, so once we restart the application, the @@no_of_users is reset
back to 0.
My question is how we can persist the class variable @@no_of_users in
the database…
Any thoughts?
Cheers,
Ganesh G…
I can see why Ganesh is thinking this - class variables will persist
over requests if you’re using something like mongrel as you’ll be
running from the same ruby instance. However, on restarting mongrel
the ruby environment is reloaded, knocking out the class variables.
For this reason, using class would be inaccurate. Most sites will be
running on more than one ruby instance meaning that the count will be
out of whack.
Class variables are ok for un-changing config vars but not suitable
for this task. As suggested, I’d use count.
Steve
I get the feeling the OP is coming from the Java / Hibernate realm. In
Rails, ActiveRecord objects are not persisted across requests, thus you
can’t do what you’re talking about here. And frankly, I’m with Bryan,
why
aren’t you just using #count?
Jason
Bryan D. wrote:
Maybe dumb question… why not just use count? That’ll be up to date.
Hi Guys,
The example I gave was just for illustration.
As you guys pointed out, we can just use User.count in this example. But
that is not my question here. What if I am crazy and actually wanted to
persist a class variable in the database. Wondering how other ORM’s
handle class variables…
Cheers,
Ganesh G…
On 4/26/07, Ganesh G. [email protected] wrote:
that is not my question here. What if I am crazy and actually wanted to
persist a class variable in the database. Wondering how other ORM’s
handle class variables…Cheers,
Ganesh G…
Well, the answer is: you don’t. I don’t think a single ORM will let you
do
this, as it just doesn’t make any sense.
Jason
Jason R. wrote:
Well, the answer is: you don’t. I don’t think a single ORM will let you
do this, as it just doesn’t make any sense.Jason
I think what Jason’s getting at is this: ORM means Object relational
mapping… it’s about mapping the state of an object to a row in the
db. A class variable is not part of an object’s state, so it doesn’t
make sense to map into a table holding object data.
If you find yourself needing to persist information about all objects of
a given class, them maybe there’s a new model there.
However, in the case of counting, yeah that’s just information you can
observe from the state of the table… i.e. a count query.
If you’re concerned about the performance of count queries, rails has an
automagic column you can add to your model/table to cache that
information.
b
Ganesh,
I think the difference is that in Rails there is not really an attempt
to provide a generic ORM solution. Rails is not objects first with a
database as an afterthought or just a way to save in-mrmory objects to
disk. Mostly those solutions fail anyway (in my experience). The
Rails mind-set is that data is stored in a database and the Rails
layer does not attempt to hide that or to provide a complete
persistence model for everything you can do in Ruby. Rails provides a
Ruby layer to access the relational database. The difference in mind-
set is a bit subtile but important. Rails does not hide SQL or try to
hide the database. It just provides a layer to make that access
easier for common cases. The storage model is relational not objects.
Michael
On Apr 26, 2:21 pm, Ganesh G. <rails-mailing-l…@andreas-
Ben M. wrote:
I think what Jason’s getting at is this: ORM means Object relational
mapping… it’s about mapping the state of an object to a row in the
db. A class variable is not part of an object’s state, so it doesn’t
make sense to map into a table holding object data.
Well, A class is an object in ruby 
Getting the point, thanks guys…
Cheers,
Ganesh G…
Sorry, hit “send” to soon.
Also, I need to correct one of your deductions. The attributes on an
ActiveRecord object are not instance variables. So your example:
ex: user = User.create(:name => “gg”, :age => 25)
p user.name #=> “gg”
p user.attributes[“name”] #=> “gg”
User does not have @name and @age. Insead, user.name ends up calling
#method_missing which ActiveRecord uses to search the @attributes hash
for
the appropriate key and value. So instance variables are not kept around
at
all, meaning you can add variables to your model without worrying about
ActiveRecord or the database engine complaining about “unknown column”.
Jason
It sounds like you have a misconception of database persistence. An
instance of a model object represents a database table row. Think of
the class as the table column headers:
id name password
1 Bill 78d877s8d
2 Steve 8873jdi873837
Now how would you write SQL to store data in the headers (table
definition)?
Let us say I need to track the number of users created. So basically I
have to maintain the number of users value in the Class level (I
understand that we can also store it in a separated model/table, but
storing it in the class level makes more sense.)
So from a model-relational mapping system this actually doesn’t make
since.
Things that are persisted in the database are instances of subclasses
of ActiveRecord::Base. User is a subclass of ActiveRecord::Base, however
User itself is an instance of Class, not AR:Base, and so can’t be
persisted using active record.
If you do want to save that sort of info add a new model for that
purpose.
Fred