Creating a many to many relationship in Rails
This is a simplified example of what I am trying to achieve, I'm relatively new to Rails and am struggling to get my head around relationships between models.
I have two models, the User
model and the Category
model. A user can be associated with many categories. A particular category can appear in the category list for many users. If a particular category is deleted, this should be reflected in the category list for a user.
In this example:
My Categories
table contains five categories:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | Sports |
| 2 | News |
| 3 | Entertainment |
| 4 | Technology |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My Users
table contains two users:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | UserA |
| 2 | UserB |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
UserA may choose Sports and Technology as his categories
UserB may choose News, Sports and Entertainment
The sports category is deleted, both UserA and UserB category lists reflect the deletion
I've toyed around with creating a UserCategories
table which holds the ids of both a category and user. This kind of worked, I could look up the category names but I couldn't get a cascading delete to work and the whole solution just seemed wrong.
The examples of using the belongs_to and has_many functions that I have found seem to discuss mapping a one-to-one relationship. For example, comments on a blog post.
- How do you represent this many-to-many relationship using the built-in Rails functionality?
- Is using a separate table between the two a viable solution when using Rails?
ruby-on-rails
add a comment |
This is a simplified example of what I am trying to achieve, I'm relatively new to Rails and am struggling to get my head around relationships between models.
I have two models, the User
model and the Category
model. A user can be associated with many categories. A particular category can appear in the category list for many users. If a particular category is deleted, this should be reflected in the category list for a user.
In this example:
My Categories
table contains five categories:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | Sports |
| 2 | News |
| 3 | Entertainment |
| 4 | Technology |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My Users
table contains two users:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | UserA |
| 2 | UserB |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
UserA may choose Sports and Technology as his categories
UserB may choose News, Sports and Entertainment
The sports category is deleted, both UserA and UserB category lists reflect the deletion
I've toyed around with creating a UserCategories
table which holds the ids of both a category and user. This kind of worked, I could look up the category names but I couldn't get a cascading delete to work and the whole solution just seemed wrong.
The examples of using the belongs_to and has_many functions that I have found seem to discuss mapping a one-to-one relationship. For example, comments on a blog post.
- How do you represent this many-to-many relationship using the built-in Rails functionality?
- Is using a separate table between the two a viable solution when using Rails?
ruby-on-rails
Actually, there's nothing wrong with usingUserCategory
model, it's even desirable in most cases. You just need to useUser.has_many :categories, through: :user_categories
. Maybe, a better name could be found though.
– RocketR
Jul 20 '12 at 13:25
For others, there is a good rails cast here about the subject: railscasts.com/episodes/47-two-many-to-many
– Dofs
Jan 26 '13 at 14:31
add a comment |
This is a simplified example of what I am trying to achieve, I'm relatively new to Rails and am struggling to get my head around relationships between models.
I have two models, the User
model and the Category
model. A user can be associated with many categories. A particular category can appear in the category list for many users. If a particular category is deleted, this should be reflected in the category list for a user.
In this example:
My Categories
table contains five categories:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | Sports |
| 2 | News |
| 3 | Entertainment |
| 4 | Technology |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My Users
table contains two users:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | UserA |
| 2 | UserB |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
UserA may choose Sports and Technology as his categories
UserB may choose News, Sports and Entertainment
The sports category is deleted, both UserA and UserB category lists reflect the deletion
I've toyed around with creating a UserCategories
table which holds the ids of both a category and user. This kind of worked, I could look up the category names but I couldn't get a cascading delete to work and the whole solution just seemed wrong.
The examples of using the belongs_to and has_many functions that I have found seem to discuss mapping a one-to-one relationship. For example, comments on a blog post.
- How do you represent this many-to-many relationship using the built-in Rails functionality?
- Is using a separate table between the two a viable solution when using Rails?
ruby-on-rails
This is a simplified example of what I am trying to achieve, I'm relatively new to Rails and am struggling to get my head around relationships between models.
I have two models, the User
model and the Category
model. A user can be associated with many categories. A particular category can appear in the category list for many users. If a particular category is deleted, this should be reflected in the category list for a user.
In this example:
My Categories
table contains five categories:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | Sports |
| 2 | News |
| 3 | Entertainment |
| 4 | Technology |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
My Users
table contains two users:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| ID | Name |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| 1 | UserA |
| 2 | UserB |
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
UserA may choose Sports and Technology as his categories
UserB may choose News, Sports and Entertainment
The sports category is deleted, both UserA and UserB category lists reflect the deletion
I've toyed around with creating a UserCategories
table which holds the ids of both a category and user. This kind of worked, I could look up the category names but I couldn't get a cascading delete to work and the whole solution just seemed wrong.
The examples of using the belongs_to and has_many functions that I have found seem to discuss mapping a one-to-one relationship. For example, comments on a blog post.
- How do you represent this many-to-many relationship using the built-in Rails functionality?
- Is using a separate table between the two a viable solution when using Rails?
ruby-on-rails
ruby-on-rails
edited Jul 18 '17 at 22:14
coreyward
50k1596124
50k1596124
asked Feb 25 '11 at 17:46
fletcherfletcher
9,24864465
9,24864465
Actually, there's nothing wrong with usingUserCategory
model, it's even desirable in most cases. You just need to useUser.has_many :categories, through: :user_categories
. Maybe, a better name could be found though.
– RocketR
Jul 20 '12 at 13:25
For others, there is a good rails cast here about the subject: railscasts.com/episodes/47-two-many-to-many
– Dofs
Jan 26 '13 at 14:31
add a comment |
Actually, there's nothing wrong with usingUserCategory
model, it's even desirable in most cases. You just need to useUser.has_many :categories, through: :user_categories
. Maybe, a better name could be found though.
– RocketR
Jul 20 '12 at 13:25
For others, there is a good rails cast here about the subject: railscasts.com/episodes/47-two-many-to-many
– Dofs
Jan 26 '13 at 14:31
Actually, there's nothing wrong with using
UserCategory
model, it's even desirable in most cases. You just need to use User.has_many :categories, through: :user_categories
. Maybe, a better name could be found though.– RocketR
Jul 20 '12 at 13:25
Actually, there's nothing wrong with using
UserCategory
model, it's even desirable in most cases. You just need to use User.has_many :categories, through: :user_categories
. Maybe, a better name could be found though.– RocketR
Jul 20 '12 at 13:25
For others, there is a good rails cast here about the subject: railscasts.com/episodes/47-two-many-to-many
– Dofs
Jan 26 '13 at 14:31
For others, there is a good rails cast here about the subject: railscasts.com/episodes/47-two-many-to-many
– Dofs
Jan 26 '13 at 14:31
add a comment |
2 Answers
2
active
oldest
votes
You want a has_and_belongs_to_many
relationship. The guide does a great job of describing how this works with charts and everything:
http://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association
You will end up with something like this:
# app/models/category.rb
class Category < ActiveRecord::Base
has_and_belongs_to_many :users
end
# app/models/user.rb
class User < ActiveRecord::Base
has_and_belongs_to_many :categories
end
Now you need to create a join table for Rails to use. Rails will not do this automatically for you. This is effectively a table with a reference to each of Categories and Users, and no primary key.
Generate a migration from the CLI like this:
bin/rails g migration CreateCategoriesUsersJoinTable
Then open it up and edit it to match:
For Rails 4.0.2+ (including Rails 5.2):
def change
# This is enough; you don't need to worry about order
create_join_table :categories, :users
# If you want to add an index for faster querying through this join:
create_join_table :categories, :users do |t|
t.index :category_id
t.index :user_id
end
end
Rails < 4.0.2:
def self.up
# Model names in alphabetical order (e.g. a_b)
create_table :categories_users, :id => false do |t|
t.integer :category_id
t.integer :user_id
end
add_index :categories_users, [:category_id, :user_id]
end
def self.down
drop_table :categories_users
end
With that in place, run your migrations and you can connect Categories and Users with all of the convenient accessors you're used to:
User.categories #=> [<Category @name="Sports">, ...]
Category.users #=> [<User @name="UserA">, ...]
User.categories.empty?
9
One additional tip: create a composite index for the table. More here stackoverflow.com/a/4381282/14540
– kolrie
Feb 26 '13 at 21:47
1
@kolrie Great point; I added to the example here for the sake of being thorough. Cheers!
– coreyward
Feb 27 '13 at 5:04
4
dumb question, does this mean it's necessary to create a new model?
– tvieira
Jan 9 '14 at 20:20
has_and_belongs_to_many does not need/create model. If you need to work directly with the relation or need to add attributes to the relationship, you should use has_many :through option. Follow the link in the answer for detailed info.
– Chong Yu
Sep 29 '14 at 19:53
12
Another thing I noticed is the naming convention of the joining table. It has to be alphabetically ordered. Using the above example, the table name has to be categories_users and not users_categories. "c" before "u" in this case.
– Chong Yu
Sep 29 '14 at 19:55
add a comment |
Just complementing coreyward's answer above:
If you already have a model that has a belongs_to
, has_many
relation and you want to create a new relation has_and_belongs_to_many
using the same table you will need to:
rails g migration CreateJoinTableUsersCategories users categories
Then,
rake db:migrate
After that, you will need to define your relations:
User.rb:
class Region < ApplicationRecord
has_and_belongs_to_many :categories
end
Category.rb
class Facility < ApplicationRecord
has_and_belongs_to_many :users
end
In order to populate the new join table with the old data, you will need to in your console:
User.all.find_each do |u|
Category.where(user_id: u.id).find_each do |c|
u.categories << c
end
end
You can either leave the user_id
column and category_id
column from the Category and User tables or create a migration to delete it.
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
You want a has_and_belongs_to_many
relationship. The guide does a great job of describing how this works with charts and everything:
http://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association
You will end up with something like this:
# app/models/category.rb
class Category < ActiveRecord::Base
has_and_belongs_to_many :users
end
# app/models/user.rb
class User < ActiveRecord::Base
has_and_belongs_to_many :categories
end
Now you need to create a join table for Rails to use. Rails will not do this automatically for you. This is effectively a table with a reference to each of Categories and Users, and no primary key.
Generate a migration from the CLI like this:
bin/rails g migration CreateCategoriesUsersJoinTable
Then open it up and edit it to match:
For Rails 4.0.2+ (including Rails 5.2):
def change
# This is enough; you don't need to worry about order
create_join_table :categories, :users
# If you want to add an index for faster querying through this join:
create_join_table :categories, :users do |t|
t.index :category_id
t.index :user_id
end
end
Rails < 4.0.2:
def self.up
# Model names in alphabetical order (e.g. a_b)
create_table :categories_users, :id => false do |t|
t.integer :category_id
t.integer :user_id
end
add_index :categories_users, [:category_id, :user_id]
end
def self.down
drop_table :categories_users
end
With that in place, run your migrations and you can connect Categories and Users with all of the convenient accessors you're used to:
User.categories #=> [<Category @name="Sports">, ...]
Category.users #=> [<User @name="UserA">, ...]
User.categories.empty?
9
One additional tip: create a composite index for the table. More here stackoverflow.com/a/4381282/14540
– kolrie
Feb 26 '13 at 21:47
1
@kolrie Great point; I added to the example here for the sake of being thorough. Cheers!
– coreyward
Feb 27 '13 at 5:04
4
dumb question, does this mean it's necessary to create a new model?
– tvieira
Jan 9 '14 at 20:20
has_and_belongs_to_many does not need/create model. If you need to work directly with the relation or need to add attributes to the relationship, you should use has_many :through option. Follow the link in the answer for detailed info.
– Chong Yu
Sep 29 '14 at 19:53
12
Another thing I noticed is the naming convention of the joining table. It has to be alphabetically ordered. Using the above example, the table name has to be categories_users and not users_categories. "c" before "u" in this case.
– Chong Yu
Sep 29 '14 at 19:55
add a comment |
You want a has_and_belongs_to_many
relationship. The guide does a great job of describing how this works with charts and everything:
http://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association
You will end up with something like this:
# app/models/category.rb
class Category < ActiveRecord::Base
has_and_belongs_to_many :users
end
# app/models/user.rb
class User < ActiveRecord::Base
has_and_belongs_to_many :categories
end
Now you need to create a join table for Rails to use. Rails will not do this automatically for you. This is effectively a table with a reference to each of Categories and Users, and no primary key.
Generate a migration from the CLI like this:
bin/rails g migration CreateCategoriesUsersJoinTable
Then open it up and edit it to match:
For Rails 4.0.2+ (including Rails 5.2):
def change
# This is enough; you don't need to worry about order
create_join_table :categories, :users
# If you want to add an index for faster querying through this join:
create_join_table :categories, :users do |t|
t.index :category_id
t.index :user_id
end
end
Rails < 4.0.2:
def self.up
# Model names in alphabetical order (e.g. a_b)
create_table :categories_users, :id => false do |t|
t.integer :category_id
t.integer :user_id
end
add_index :categories_users, [:category_id, :user_id]
end
def self.down
drop_table :categories_users
end
With that in place, run your migrations and you can connect Categories and Users with all of the convenient accessors you're used to:
User.categories #=> [<Category @name="Sports">, ...]
Category.users #=> [<User @name="UserA">, ...]
User.categories.empty?
9
One additional tip: create a composite index for the table. More here stackoverflow.com/a/4381282/14540
– kolrie
Feb 26 '13 at 21:47
1
@kolrie Great point; I added to the example here for the sake of being thorough. Cheers!
– coreyward
Feb 27 '13 at 5:04
4
dumb question, does this mean it's necessary to create a new model?
– tvieira
Jan 9 '14 at 20:20
has_and_belongs_to_many does not need/create model. If you need to work directly with the relation or need to add attributes to the relationship, you should use has_many :through option. Follow the link in the answer for detailed info.
– Chong Yu
Sep 29 '14 at 19:53
12
Another thing I noticed is the naming convention of the joining table. It has to be alphabetically ordered. Using the above example, the table name has to be categories_users and not users_categories. "c" before "u" in this case.
– Chong Yu
Sep 29 '14 at 19:55
add a comment |
You want a has_and_belongs_to_many
relationship. The guide does a great job of describing how this works with charts and everything:
http://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association
You will end up with something like this:
# app/models/category.rb
class Category < ActiveRecord::Base
has_and_belongs_to_many :users
end
# app/models/user.rb
class User < ActiveRecord::Base
has_and_belongs_to_many :categories
end
Now you need to create a join table for Rails to use. Rails will not do this automatically for you. This is effectively a table with a reference to each of Categories and Users, and no primary key.
Generate a migration from the CLI like this:
bin/rails g migration CreateCategoriesUsersJoinTable
Then open it up and edit it to match:
For Rails 4.0.2+ (including Rails 5.2):
def change
# This is enough; you don't need to worry about order
create_join_table :categories, :users
# If you want to add an index for faster querying through this join:
create_join_table :categories, :users do |t|
t.index :category_id
t.index :user_id
end
end
Rails < 4.0.2:
def self.up
# Model names in alphabetical order (e.g. a_b)
create_table :categories_users, :id => false do |t|
t.integer :category_id
t.integer :user_id
end
add_index :categories_users, [:category_id, :user_id]
end
def self.down
drop_table :categories_users
end
With that in place, run your migrations and you can connect Categories and Users with all of the convenient accessors you're used to:
User.categories #=> [<Category @name="Sports">, ...]
Category.users #=> [<User @name="UserA">, ...]
User.categories.empty?
You want a has_and_belongs_to_many
relationship. The guide does a great job of describing how this works with charts and everything:
http://guides.rubyonrails.org/association_basics.html#the-has-and-belongs-to-many-association
You will end up with something like this:
# app/models/category.rb
class Category < ActiveRecord::Base
has_and_belongs_to_many :users
end
# app/models/user.rb
class User < ActiveRecord::Base
has_and_belongs_to_many :categories
end
Now you need to create a join table for Rails to use. Rails will not do this automatically for you. This is effectively a table with a reference to each of Categories and Users, and no primary key.
Generate a migration from the CLI like this:
bin/rails g migration CreateCategoriesUsersJoinTable
Then open it up and edit it to match:
For Rails 4.0.2+ (including Rails 5.2):
def change
# This is enough; you don't need to worry about order
create_join_table :categories, :users
# If you want to add an index for faster querying through this join:
create_join_table :categories, :users do |t|
t.index :category_id
t.index :user_id
end
end
Rails < 4.0.2:
def self.up
# Model names in alphabetical order (e.g. a_b)
create_table :categories_users, :id => false do |t|
t.integer :category_id
t.integer :user_id
end
add_index :categories_users, [:category_id, :user_id]
end
def self.down
drop_table :categories_users
end
With that in place, run your migrations and you can connect Categories and Users with all of the convenient accessors you're used to:
User.categories #=> [<Category @name="Sports">, ...]
Category.users #=> [<User @name="UserA">, ...]
User.categories.empty?
edited Nov 19 '18 at 16:30
answered Feb 25 '11 at 17:48
coreywardcoreyward
50k1596124
50k1596124
9
One additional tip: create a composite index for the table. More here stackoverflow.com/a/4381282/14540
– kolrie
Feb 26 '13 at 21:47
1
@kolrie Great point; I added to the example here for the sake of being thorough. Cheers!
– coreyward
Feb 27 '13 at 5:04
4
dumb question, does this mean it's necessary to create a new model?
– tvieira
Jan 9 '14 at 20:20
has_and_belongs_to_many does not need/create model. If you need to work directly with the relation or need to add attributes to the relationship, you should use has_many :through option. Follow the link in the answer for detailed info.
– Chong Yu
Sep 29 '14 at 19:53
12
Another thing I noticed is the naming convention of the joining table. It has to be alphabetically ordered. Using the above example, the table name has to be categories_users and not users_categories. "c" before "u" in this case.
– Chong Yu
Sep 29 '14 at 19:55
add a comment |
9
One additional tip: create a composite index for the table. More here stackoverflow.com/a/4381282/14540
– kolrie
Feb 26 '13 at 21:47
1
@kolrie Great point; I added to the example here for the sake of being thorough. Cheers!
– coreyward
Feb 27 '13 at 5:04
4
dumb question, does this mean it's necessary to create a new model?
– tvieira
Jan 9 '14 at 20:20
has_and_belongs_to_many does not need/create model. If you need to work directly with the relation or need to add attributes to the relationship, you should use has_many :through option. Follow the link in the answer for detailed info.
– Chong Yu
Sep 29 '14 at 19:53
12
Another thing I noticed is the naming convention of the joining table. It has to be alphabetically ordered. Using the above example, the table name has to be categories_users and not users_categories. "c" before "u" in this case.
– Chong Yu
Sep 29 '14 at 19:55
9
9
One additional tip: create a composite index for the table. More here stackoverflow.com/a/4381282/14540
– kolrie
Feb 26 '13 at 21:47
One additional tip: create a composite index for the table. More here stackoverflow.com/a/4381282/14540
– kolrie
Feb 26 '13 at 21:47
1
1
@kolrie Great point; I added to the example here for the sake of being thorough. Cheers!
– coreyward
Feb 27 '13 at 5:04
@kolrie Great point; I added to the example here for the sake of being thorough. Cheers!
– coreyward
Feb 27 '13 at 5:04
4
4
dumb question, does this mean it's necessary to create a new model?
– tvieira
Jan 9 '14 at 20:20
dumb question, does this mean it's necessary to create a new model?
– tvieira
Jan 9 '14 at 20:20
has_and_belongs_to_many does not need/create model. If you need to work directly with the relation or need to add attributes to the relationship, you should use has_many :through option. Follow the link in the answer for detailed info.
– Chong Yu
Sep 29 '14 at 19:53
has_and_belongs_to_many does not need/create model. If you need to work directly with the relation or need to add attributes to the relationship, you should use has_many :through option. Follow the link in the answer for detailed info.
– Chong Yu
Sep 29 '14 at 19:53
12
12
Another thing I noticed is the naming convention of the joining table. It has to be alphabetically ordered. Using the above example, the table name has to be categories_users and not users_categories. "c" before "u" in this case.
– Chong Yu
Sep 29 '14 at 19:55
Another thing I noticed is the naming convention of the joining table. It has to be alphabetically ordered. Using the above example, the table name has to be categories_users and not users_categories. "c" before "u" in this case.
– Chong Yu
Sep 29 '14 at 19:55
add a comment |
Just complementing coreyward's answer above:
If you already have a model that has a belongs_to
, has_many
relation and you want to create a new relation has_and_belongs_to_many
using the same table you will need to:
rails g migration CreateJoinTableUsersCategories users categories
Then,
rake db:migrate
After that, you will need to define your relations:
User.rb:
class Region < ApplicationRecord
has_and_belongs_to_many :categories
end
Category.rb
class Facility < ApplicationRecord
has_and_belongs_to_many :users
end
In order to populate the new join table with the old data, you will need to in your console:
User.all.find_each do |u|
Category.where(user_id: u.id).find_each do |c|
u.categories << c
end
end
You can either leave the user_id
column and category_id
column from the Category and User tables or create a migration to delete it.
add a comment |
Just complementing coreyward's answer above:
If you already have a model that has a belongs_to
, has_many
relation and you want to create a new relation has_and_belongs_to_many
using the same table you will need to:
rails g migration CreateJoinTableUsersCategories users categories
Then,
rake db:migrate
After that, you will need to define your relations:
User.rb:
class Region < ApplicationRecord
has_and_belongs_to_many :categories
end
Category.rb
class Facility < ApplicationRecord
has_and_belongs_to_many :users
end
In order to populate the new join table with the old data, you will need to in your console:
User.all.find_each do |u|
Category.where(user_id: u.id).find_each do |c|
u.categories << c
end
end
You can either leave the user_id
column and category_id
column from the Category and User tables or create a migration to delete it.
add a comment |
Just complementing coreyward's answer above:
If you already have a model that has a belongs_to
, has_many
relation and you want to create a new relation has_and_belongs_to_many
using the same table you will need to:
rails g migration CreateJoinTableUsersCategories users categories
Then,
rake db:migrate
After that, you will need to define your relations:
User.rb:
class Region < ApplicationRecord
has_and_belongs_to_many :categories
end
Category.rb
class Facility < ApplicationRecord
has_and_belongs_to_many :users
end
In order to populate the new join table with the old data, you will need to in your console:
User.all.find_each do |u|
Category.where(user_id: u.id).find_each do |c|
u.categories << c
end
end
You can either leave the user_id
column and category_id
column from the Category and User tables or create a migration to delete it.
Just complementing coreyward's answer above:
If you already have a model that has a belongs_to
, has_many
relation and you want to create a new relation has_and_belongs_to_many
using the same table you will need to:
rails g migration CreateJoinTableUsersCategories users categories
Then,
rake db:migrate
After that, you will need to define your relations:
User.rb:
class Region < ApplicationRecord
has_and_belongs_to_many :categories
end
Category.rb
class Facility < ApplicationRecord
has_and_belongs_to_many :users
end
In order to populate the new join table with the old data, you will need to in your console:
User.all.find_each do |u|
Category.where(user_id: u.id).find_each do |c|
u.categories << c
end
end
You can either leave the user_id
column and category_id
column from the Category and User tables or create a migration to delete it.
answered Jan 21 at 22:10
Bruno MBruno M
214
214
add a comment |
add a comment |
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Actually, there's nothing wrong with using
UserCategory
model, it's even desirable in most cases. You just need to useUser.has_many :categories, through: :user_categories
. Maybe, a better name could be found though.– RocketR
Jul 20 '12 at 13:25
For others, there is a good rails cast here about the subject: railscasts.com/episodes/47-two-many-to-many
– Dofs
Jan 26 '13 at 14:31