Can i eager-load associations onto an already-loaded object?

Rails
1

Let’s say that i have a User object in @user, already loaded. I want
to constantly refer to @user.role on a particular page (where User
belongs_to :role), so i want to ‘eager load’ the role association.

For the sake of argument, let’s say i wasn’t able to do this in the
normal way (ie with the :include option to a find call) when i loaded
the user out of the database in the first place, but need to do it
subsequent to that. Is that possible?

2

Max W. wrote:

Let’s say that i have a User object in @user, already loaded. I want
to constantly refer to @user.role on a particular page (where User
belongs_to :role), so i want to ‘eager load’ the role association.

For the sake of argument, let’s say i wasn’t able to do this in the
normal way (ie with the :include option to a find call) when i loaded
the user out of the database in the first place, but need to do it
subsequent to that. Is that possible?

Just to answer my own question, sort of, i think that i can do

@user[:role] = @user.role

and that sets the instance variable which is used for subsequent calls
to @user.role, effectively eager loading that association. is that
correct? Is this the same thing as the eager loading would do? eg, is
it precisely as prone to weird side effects as the regular eager
loading?

3

On Jun 8, 11:25 am, Max W. [email protected] wrote:

Just to answer my own question, sort of, i think that i can do

@user[:role] = @user.role

and that sets the instance variable which is used for subsequent calls
to @user.role, effectively eager loading that association. is that
correct? Is this the same thing as the eager loading would do? eg, is
it precisely as prone to weird side effects as the regular eager
loading?

It’s not at all what normal eager loading would do.

if you do

class User < ActiveRecord::Base
class << self
public :preload_associations
end
end

then you can do

User.preload_associations [user1, user2, user3], :role

(at least in rails 2.x - might very easily not work in 3.x)

Fred

Fred

4

On Jun 8, 11:41 am, Frederick C. [email protected]
wrote:

On Jun 8, 11:25 am, Max W. [email protected] wrote:
then you can do

User.preload_associations [user1, user2, user3], :role

One addition - it’s pointless doing this unless you have more than 1
user object - it won’t be any faster

Fred

5

Frederick C. wrote:

One addition - it’s pointless doing this unless you have more than 1
user object - it won’t be any faster

Fred

Thanks fred - that works. Faster than what, though? Than my approach?

6

On Jun 8, 12:09 pm, Max W. [email protected] wrote:

Frederick C. wrote:

One addition - it’s pointless doing this unless you have more than 1
user object - it won’t be any faster

Fred

Thanks fred - that works. Faster than what, though? Than my approach?

compared to not eagerloading it at all.

Fred

7

On 8 June 2010 14:21, Max W. [email protected] wrote:

In this case, i have a page which loads a partial twenty times. Each
partial calls a method on the user object (current_user as it happens)
which inspects its associated role object. So, if i don’t eager load,
then the role gets loaded twenty times. I might be misunderstanding
your point though.

Memoize the role if necessary (although I thought AR would do that for
you after the first load - not got time to check it out to be sure at
the moment)

8

Frederick C. wrote:

On Jun 8, 12:09�pm, Max W. [email protected] wrote:

Frederick C. wrote:

One addition - it’s pointless doing this unless you have more than 1
user object - it won’t be any faster

Fred

Thanks fred - that works. �Faster than what, though? �Than my approach?

compared to not eagerloading it at all.

Fred

In this case, i have a page which loads a partial twenty times. Each
partial calls a method on the user object (current_user as it happens)
which inspects its associated role object. So, if i don’t eager load,
then the role gets loaded twenty times. I might be misunderstanding
your point though.

9

On Jun 8, 2:21 pm, Max W. [email protected] wrote:

Fred

In this case, i have a page which loads a partial twenty times. Each
partial calls a method on the user object (current_user as it happens)
which inspects its associated role object. So, if i don’t eager load,
then the role gets loaded twenty times. I might be misunderstanding
your point though.

If it’s the same user object throughout active record will cache the
loaded role object for you - no need for eager loading here.

Fred

10

Frederick C. wrote:

On Jun 8, 2:21�pm, Max W. [email protected] wrote:

Fred

In this case, i have a page which loads a partial twenty times. �Each
partial calls a method on the user object (current_user as it happens)
which inspects its associated role object. So, if i don’t eager load,
then the role gets loaded twenty times. �I might be misunderstanding
your point though.

If it’s the same user object throughout active record will cache the
loaded role object for you - no need for eager loading here.

Fred

ah i see, i knew mysql cached the repeated queries but didn’t know AR
cached the retrieved association as well.

thanks! max