Ruby on Rails Associations
Overview
The era of technology is moving forward at an incredibly swift speed, and as a result, how we engage with and store data are becoming increasingly difficult.
Consider, for instance, in a social media post it could have a connection with the individual who created it, the remarks on the post, the comments and shares it receives, and even the place and moment it was posted. Each of these individual bits of information can offer valuable insights about the post, and by assessing them collectively, we can obtain a more comprehensive analysis of data. To easily establish relations and collectively work with this type of data, we use rails associations in our Rails application.
Ruby on Rails Associations Introduction
Rails associations play an important role in establishing interconnectivity between different parts of the application's data. Each data item is stored in an SQL table with its unique identifier and relationship information with other tables. To establish these relationships, we can use rails associations to combine information from multiple tables.
There are different types of data relationships such as one-to-one, many-to-one, and many-to-many. When creating association rails, the relationship type directly affects how SQL queries are written and executed. We can use built-in methods to easily rebind models and connect their data in meaningful ways.
Benefits of Using Rails Associations
- Simplified representation: Rails associations provide an easy-to-understand way to represent relationships between models in code.
- Fewer SQL queries: By reducing the number of SQL queries, efficiency is improved and less code is needed to manage relationships between models.
- Consistency: Track association work to ensure relationships between models are consistent across the application(useful in complex data joins).
- Flexibility: Easily manage relationships between models. Any changes made to the relationships between models will not affect the rest of the application.
Types of Rails Associations in Ruby on Rails
There is a total of six rails Associations:
1.belongs_to Association
In rails associations, the belongs_to association allows developers to specify a connection between two models, establishing either a one-to-one or a many-to-one relationship within the application.
Let's take an example of a belongs_to association between the Post and User models:
The Post model is connected to a User model via the belongs_to association. This relationship is established by the user_id column that is present in each post, which stores the ID of the associated user.
- class_name: This option specifies the name of the associated model when it cannot be inferred from the association name. Example:
- foreign_key: This option specifies the name of the foreign key column used to create the association. Example:
- primary_key: This option specifies the name of the primary key column in the associated table. Example:
2. has_one Association
In rails associations, to establish a one-to-one relationship with another model, we can use the has_one association. This allows each instance of the declaring model to contain one instance of the other model. As an example, a User model may be associated with one Profile model.
- class_name: Each instance of the User class has one associated instance of the Profile class, which is referred to as user_profile.
Example:
- foreign_key: It specifies the name of the foreign key column used in the association, which in this case is 'user_id'.
Example:
- dependent: To specify the behavior of the association when the associated record is deleted. Example:
3. has_many Association
has_many association provides one-to-many relationship between two models. Each instance of the model has zero or more instances of another model.
Let's say we have two models User model and the Post model. Each user can have multiple posts.
Note: The association name is plural because a user can have many posts.
- class_name: used to specify the name of the model class when it cannot be inferred from the association name. For example:
- foreign_key: used to set the name of the foreign key column on the other model if it is not the default of model_name_id. For example:
- primary_key: used to set the primary key of the associated model if it is not the default of id. For example:
4. has_and_belongs_to_many Association
In rails, associations, has_and_belongs_to_many is used to establish a many-to-many relationship between two models. Each instance of one model can be associated with many instances of another model, and vice versa.
Let's say we have two models, a User model, and a Group model. A user can belong to multiple groups and a group can have multiple users.
- class_name: used to specify the name of the model class when it cannot be inferred from the association name. For example:
- join_table: used to set the name of the join table that is used to associate the two models. For example:
- foreign_key: used to set the name of the foreign key column on the join table if it is not the default of model_name_id. For example:
- association_foreign_key: used to set the name of the foreign key column on the join table for the associated model if it is not the default of associated_model_name_id. For example:
5. has_many: through association
The has_many: through association provides many-to-many relations between two models through a third model. It's also useful when we need to store additional information about that relationship.
For example, we have 'User' and 'Group' models in our app. Each user can belong to multiple groups, and each group can have multiple users. To keep track of when each user joins each group, we can create a third model called 'Membership'. 'Membership' would have foreign keys pointing to both 'User' and 'Group' models.
6. has_one :through association
The 'has_one: through' association provides a one-to-one association between two models through a third model. It's useful when a direct association between two models is not possible, or we need additional attributes to be stored on the third model.
Let's say we have three models: User model - application's users. Subscription model - user's subscription to the service. Plan model - different subscription plans available.
To set up one-to-one 'has_one
Polymorphic Associations
In rails associations, polymorphic associations allow models to belong to multiple other models. For example, a comment can be associated with a post or a photo. Let's say we have three models, Comment, Image, and Post. We want to allow comments to be associated with either a post or an image:
FAQs
Q: What is the difference between belongs_to and has_many associations?
A: A belongs_to association specifies that a particular record "belongs to" another record, while a has_many association specifies that a record "has many" other records.
Q: Can we have multiple associations between the two models?
A: Yes, we can have multiple associations between the two models. For example, a user might have many blog posts, but might also have many comments on other users' blog posts.
Q: What is a through association?
A: A through association allows us to define a relationship between two models through a third model.
Conclusion
- Different types of data relationships such as one-to-one, one-to-many, and many-to-many can be implemented through associations rails.
- In Rails, 6 types of associations are possible:
- belongs_to
- has_one
- has_many
- has_and_belongs_to_many
- has_many :through
- has_one :through
- Polymorphic associations allow models to belong to multiple other models.