With Active Record associations, we can streamline these - and other - operations by declaratively telling Rails that there is a connection between the two models. Here's the revised code for setting up authors and books:

To learn more about the different types of associations, read the next section of this guide. That's followed by some tips and tricks for working with associations, and then by a complete reference to the methods and options for associations in Rails.

Associations Download

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Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model belongs_to another, you instruct Rails to maintain Primary Key-Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model.

A belongs_to association sets up a connection with another model, such that each instance of the declaring model "belongs to" one instance of the other model. For example, if your application includes authors and books, and each book can be assigned to exactly one author, you'd declare the book model this way:

belongs_to associations must use the singular term. If you used the pluralized form in the above example for the author association in the Book model and tried to create the instance by Book.create(authors: @author), you would be told that there was an "uninitialized constant Book::Authors". This is because Rails automatically infers the class name from the association name. If the association name is wrongly pluralized, then the inferred class will be wrongly pluralized too.

When used alone, belongs_to produces a one-directional one-to-one connection. Therefore each book in the above example "knows" its author, but the authors don't know about their books.To setup a bi-directional association - use belongs_to in combination with a has_one or has_many on the other model, in this case the Author model.

belongs_to does not ensure reference consistency if optional is set to true, so depending on the use case, you might also need to add a database-level foreign key constraint on the reference column, like this:

Depending on the use case, you might also need to create a unique index and/ora foreign key constraint on the supplier column for the accounts table. In thiscase, the column definition might look like this:

A has_many association is similar to has_one, but indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a belongs_to association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing authors and books, the author model could be declared like this:

A has_many :through association is often used to set up a many-to-many connection with another model. This association indicates that the declaring model can be matched with zero or more instances of another model by proceeding through a third model. For example, consider a medical practice where patients make appointments to see physicians. The relevant association declarations could look like this:

The has_many :through association is also useful for setting up "shortcuts" through nested has_many associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. You could set that up this way:

A has_one :through association sets up a one-to-one connection with another model. This association indicatesthat the declaring model can be matched with one instance of another model by proceeding through a third model.For example, if each supplier has one account, and each account is associated with one account history, then thesupplier model could look like this:

A has_and_belongs_to_many association creates a direct many-to-many connection with another model, with no intervening model.This association indicates that each instance of the declaring model refers to zero or more instances of another model.For example, if your application includes assemblies and parts, with each assembly having many parts and each part appearing in many assemblies, you could declare the models this way:

The distinction is in where you place the foreign key (it goes on the table for the class declaring the belongs_to association), but you should give some thought to the actual meaning of the data as well. The has_one relationship says that one of something is yours - that is, that something points back to you. For example, it makes more sense to say that a supplier owns an account than that an account owns a supplier. This suggests that the correct relationships are like this:

The simplest rule of thumb is that you should set up a has_many :through relationship if you need to work with the relationship model as an independent entity. If you don't need to do anything with the relationship model, it may be simpler to set up a has_and_belongs_to_many relationship (though you'll need to remember to create the joining table in the database).

While has_and_belongs_to_many suggests creating a join table with no primary key via id: false, consider using a composite primary key for the join table in the has_many :through relationship.For example, it's recommended to use create_table :manifests, primary_key: [:assembly_id, :part_id] in the example above.

A slightly more advanced twist on associations is the polymorphic association. With polymorphic associations, a model can belong to more than one other model, on a single association. For example, you might have a picture model that belongs to either an employee model or a product model. Here's how this could be declared:

You can think of a polymorphic belongs_to declaration as setting up an interface that any other model can use. From an instance of the Employee model, you can retrieve a collection of pictures: @employee.pictures.

If you have an instance of the Picture model, you can get to its parent via @picture.imageable. To make this work, you need to declare both a foreign key column and a type column in the model that declares the polymorphic interface:

Here, Rails assumes that the :id column should be used as the primary key for the association between an orderand its books, just as with a regular has_many / belongs_to association. It will infer that the foreign key columnon the books table is :order_id. Accessing a book's order:

This only works if the model's composite primary key contains the :id column, and the column is unique forall records. In order to use the full composite primary key in associations, set the query_constraints option onthe association. This option specifies a composite foreign key on the association: all columns in the foreign key willbe used when querying the associated record(s). For example:

In designing a data model, you will sometimes find a model that should have a relation to itself. For example, you may want to store all employees in a single database model, but be able to trace relationships such as between manager and subordinates. This situation can be modeled with self-joining associations:

You are not free to use just any name for your associations. Because creating an association adds a method with that name to the model, it is a bad idea to give an association a name that is already used for an instance method of ActiveRecord::Base. The association method would override the base method and break things. For instance, attributes or connection are bad names for associations.

Associations are extremely useful, but they are not magic. You are responsible for maintaining your database schema to match your associations. In practice, this means two things, depending on what sort of associations you are creating. For belongs_to associations you need to create foreign keys, and for has_and_belongs_to_many associations you need to create the appropriate join table.

If you create a has_and_belongs_to_many association, you need to explicitly create the joining table. Unless the name of the join table is explicitly specified by using the :join_table option, Active Record creates the name by using the lexical order of the class names. So a join between author and book models will give the default join table name of "authors_books" because "a" outranks "b" in lexical ordering.

The precedence between model names is calculated using the operator for String. This means that if the strings are of different lengths, and the strings are equal when compared up to the shortest length, then the longer string is considered of higher lexical precedence than the shorter one. For example, one would expect the tables "paper_boxes" and "papers" to generate a join table name of "papers_paper_boxes" because of the length of the name "paper_boxes", but it in fact generates a join table name of "paper_boxes_papers" (because the underscore '_' is lexicographically less than 's' in common encodings).

We pass id: false to create_table because that table does not represent a model. That's required for the association to work properly. If you observe any strange behavior in a has_and_belongs_to_many association like mangled model IDs, or exceptions about conflicting IDs, chances are you forgot that bit.

Active Record supports automatic identification for most associations with standard names. However, bi-directional associations that contain the :through or :foreign_key options will not be automatically identified.

Custom scopes on the opposite association also prevent automatic identification, as do custom scopes on the association itself unless config.active_record.automatic_scope_inversing is set to true (the default for new applications).

In database terms, the belongs_to association says that this model's table contains a column which represents a reference to another table.This can be used to set up one-to-one or one-to-many relations, depending on the setup.If the table of the other class contains the reference in a one-to-one relation, then you should use has_one instead. 152ee80cbc

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