How do I create and use a junction table in Rails?
I have the following data:
- A post called
Hello
has categoriesgreet
- Another post called
Hola
has categoriesgreet, international
My schema is:
create_table "posts", :force => true do |t|
t.string "name"
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "categories", :force => true do |t|
t.string "name"
t.datetime "created_at"
t.datetime "updated_at"
end
create_table "posts_categories", :force => true do |t|
t.integer "post_id"
t.integer "category_id"
t.datetime "created_at"
t.datetime "updated_开发者_开发技巧at"
end
After reading the Rails guide, the most suitable relationship for the above seems to be:
class Post < ActiveRecord::Base
has_and_belongs_to_many :categories
end
class Category < ActiveRecord::Base
has_and_belongs_to_many :posts
end
My junction table also seems to have a primary key. I think I need to get rid of it.
- What's the initial migration command to generate a junction table in Rails?
- What's the best course of action, should I drop
posts_categories
and re-create it or just drop the primary key column? - Does the junction table have a corresponding model? I have used
scaffold
to generate the junction table code, should I get rid of the extra code?
Assuming all the above has been fixed and is working properly, how do I query all posts and display them along with their named categories in the view. For example:
Post #1 - hello, categories: greet
Post #2 - hola, categories: greet, international
you might want to check out this web page from the rails API doc.
the easiest way to generate a junction table is
"script/generate model categories_posts category_id:integer post_id:integer"
. Note that the class names should be in alphabetical order. I'm fairly indifferent to the whole primary key thing, but if it becomes an issue, you can generate a migration to drop like 'script/generate DropPostsCategoriesIdFromPostsCategories posts_categories_id:integer' (make sure this migration file does what you want, i haven't tested it and what it does may vary by your version of rails) and then dorake db:migrate
. to change the DB.for the class name you're using, you could use:
class Post < ActiveRecord::Base
has_many :categories, :through => :posts_categories
end
has_many through lets you specify the names of the join/junction table. or you could drop the table and regenerate it with the right name.
- (should be #3) yeah, just generate a model for a join class, don't do the entire scaffold. (see above)
to find all posts, do something like @posts = Post.find(:all)
to print out the categories, do something like
@posts.each do | post |
print post.name, "\n"
@posts.categories.each do | cat |
print "\t", cat.name, "\n"
end
end
in actual rails code, you'd want to do that in the view, this is more of a console output-type thing.
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