Model data through another model

Rails
1

Hey,

I think this is pretty simple, but I just can’t get my head around it…

I have a forum with the following models

CATEGORIES
id
title

TOPICS
id
title
category_id
user_id
created_on

POSTS
id
message
user_id
topic_id
created_on

I want to be able to display the time when the last POST was posted in
each CATEGORY.

Should i change my database layout, or is there a way to do it with my
current layout?

Thanks for your help guys!

2

Scott H. wrote:

Hey,

I think this is pretty simple, but I just can’t get my head around it…

I have a forum with the following models

CATEGORIES
id
title

TOPICS
id
title
category_id
user_id
created_on

POSTS
id
message
user_id
topic_id
created_on

I want to be able to display the time when the last POST was posted in
each CATEGORY.

Should i change my database layout, or is there a way to do it with my
current layout?

Thanks for your help guys!

You can do it with your current layout, but it will be expensive. The
best way is to create a cache on your category table. And then set that
cache in an after_create filter on the post model.

class Post
after_create :set_last_post
def set_last_post
topic.category.update_attribute(:last_post_at, Time.now)
end
end

Category.find(1).last_post_at #=> cached datetime

3

Hi Scott,

or you could use the relationship that is already built in:
@last_post = Category.find(cat_id).posts.find(:first, :order =>
“created_on DESC”)
assuming you set up the relationships and stuff.

I would probably stick that method in the Category model:

class Category

def self.find_last_post(cat_id)
  Category.find(cat_id).posts.find(:first, :order => "created_on

DESC")
end
end

then you can do:
@last_sent = Category.find_last_post(cat_id)

I think Alex’s way may be faster.

–jake

Alex W. wrote:

Scott H. wrote:

Hey,

I think this is pretty simple, but I just can’t get my head around it…

I have a forum with the following models

CATEGORIES
id
title

TOPICS
id
title
category_id
user_id
created_on

POSTS
id
message
user_id
topic_id
created_on

I want to be able to display the time when the last POST was posted in
each CATEGORY.

Should i change my database layout, or is there a way to do it with my
current layout?

Thanks for your help guys!

You can do it with your current layout, but it will be expensive. The
best way is to create a cache on your category table. And then set that
cache in an after_create filter on the post model.

class Post
after_create :set_last_post
def set_last_post
topic.category.update_attribute(:last_post_at, Time.now)
end
end

Category.find(1).last_post_at #=> cached datetime

4

cheers guys.

I’ll give them ago tonight and let you know how it goes!