Fork and clone this repository, and then run bundle install.

By the end of this lesson, students should be able to:

• Explain the relationship between constraints and validations
• Use constraints to help enforce data integrity
• Use validations to help enforce data integrity

A company's data can sometimes be its most valuable asset, more so than code or even employees. There are several different places where data integrity can be maintained within an application, including:

• within the database itself
• on the client-side (implemented in JavaScript)
• in the controllers

However, each of these approaches has advantages and disadvantages.

Rails's perspective is that the best places for dealing with data integrity are in migrations and in the model, since they have all of the advantages of controller validation but none of the disadvantages.

Except strong parameters - that kind of validation is conventionally done in the controller.

Let's look at some ways ActiveRecord helps us to maintain data integrity.

As you may recall from the SQL material, a constraint is a restriction on the data allowed in a table column (or columns). ActiveRecord Migrations allow us to define constraints on our tables from within Rails. We've already seen a few instances of this in previous projects' migration files:

class CreateCountries < ActiveRecord::Migration
  def change
    create_table :countries do |t|
      t.string :name
      t.integer :population
      t.string :language

      t.timestamps null: false # `null: false` is a constraint
    end
  end
end

Different SQL implementations use a variety of different constraints, and while ActiveRecord supports some of these, it doesn't support them all. The most important ones that it does support, across the board, are:

• null: false

  Sets the NOT NULL constraint in SQL. NOT NULL prevents the database from saving a row without a value in that particular column into the database.

  EXAMPLE : Suppose you want to ensure that every country entered has a name - blank names are forbidden. In the migration file for Countries, you can write:

  class CreateCountries < ActiveRecord::Migration
    def change
      create_table :countries do |t|
        t.string :name, null: false       # added ', null: false'
        t.integer :population
        t.string :language
  
        t.timestamps null: false
      end
    end
  end

  If you run this migation, and then open up the Rails Console, you will be unable to create new Countries without specifying names for them.

• unique: true / index: {unique: true}

  Sets the UNIQUE constraint in SQL. This can be used to define a single column as having only unique values, or to specify that certain combinations of column values must be unique; it accomplishes this by creating a special hidden index column in the database that records unique combinations of other columns.

  EXAMPLE : Consider a second model, Person. Suppose you want to ensure that each person has a unique phone number, and a unique full name (given_name + surname); you might make the following change to the CreatePeople migration.

  class CreatePeople < ActiveRecord::Migration
    def change
      create_table :people do |t|
        t.string :given_name
        t.string :surname
        t.string :phone_number
  
        t.timestamps null: false
      end
  
      add_index :people, :phone_number, unique: true
      add_index :people, [:given_name, :surname], unique: true
    end
  end

  When using unique: ___ , you will usually also want to also specify null: false. This is because a NULL value never equals anything, including itself, so NULL values are always considered unique. As such, if you don't add the null: false, your supposedly-unique column values might contain many (effectively identical) null values.

  NOTEWORTHY ASIDE

  index: <hash> works by passing on any parameters given in the hash to the add_index method.

  add_column :people, :phone_number, :string, index: {unique: true}
  # Above and below are equivalent.
  add_column :people, :phone_number, :string
  add_index :people, :phone_number, unique: true

  You may also see index: true in an add_reference method; in that context, index: true is telling Rails to create a new index column and to make that the reference.

• foreign_key: true / references: {foreign_key: true}

  Set the FOREIGN KEY constraint in SQL. As you may recall, this requires that a foreign key match an existing id in the table being referenced.

  As with the uniqueness constraint, this doesn't prevent null values in the referring column, so we'll usually want to include the null:false option.

  EXAMPLE : Now that we have 'Country' and 'Person' resources, suppose we want to link them together through a third resource called 'Citizenship'. Running rails g model Citizenship status date_obtained:date, builds a new migration file and model. You can then create two new empty migrations to link all of the tables.

  class AddPeopleToCitizenships < ActiveRecord::Migration
    def change
      add_reference :citizenships, :person, index: true, foreign_key: true
    end
  end

  class AddCountriesToCitizenships < ActiveRecord::Migration
    def change
      add_reference :citizenships, :country, index: true, foreign_key: true
    end
  end

  add_reference creates a new column with a FOREIGN KEY constaint; if you want to add the foreign key constraint to an existing column, use add_foreign_key instead of add_reference

  Once you add the appropriate methods to the models, you can test it in the Rails Console.

  class Country < ActiveRecord::Base
    has_many :citizenships
    has_many :people, through: :citizenships
  end

  class Person < ActiveRecord::Base
    has_many :citizenships
    has_many :countries, through: :citizenships
  end

  class Citizenship < ActiveRecord::Base
    belongs_to :country
    belongs_to :person
  end

In your squads, follow the example above and create three new resources that have a has_many ... , through ... relationship, and add non-null, uniqueness, and foreign key constraints to all three via the migration files.

Individually, create two new resources in the same application that exhibit a one-to-many (has_many/belongs_to) relationship.

ActiveRecord::Base provides validator methods that allow us to perform checks on model properties. For certain model methods (create, create!, save, save!, update, update!), if any requested check fails, the model cannot be saved. There are many more model validation methods than there are ways to set constraints in migration files, so you'll often see more validation done in models than in migrations.

Let's look at how we might validate the same three things that we wanted to validate above: empty values, uniqueness, and references.

• Empty Values

  To prevent empty values we'll use validates <property>, presence: true. This is a slightly more restrictive check than null: false; it disallows both empty values (nil in Ruby, NULL in the database), and it also disallows empty strings (for string properties).

  EXAMPLE : To set an empty-value validator in our Country model, you might write

  class Country < ActiveRecord::Base
    has_many :citizenships
    has_many :people, through: :citizenships
  
    validates :name, presence: true
  end

• Uniqueness

  To ensure that a property is unique, we'll use validates <property>, uniqueness: true. If this is a multi-column uniqueness check, we replace the boolean with a hash providing a scope for the uniqueness check, e.g. {scope: <other property>}.

  EXAMPLE : To set some uniqueness validators in my Person model, you might write

  class Person < ActiveRecord::Base
    has_many :citizenships
    has_many :countries, through: :citizenships
  
    validates :phone_number, uniqueness: true
    validates :given_name, uniqueness: {scope: :surname}
    validates :surname, uniqueness: {scope: :given_name}
  end

• References

  For referential integrity checks, we'll use validates <model>, presence: true, where <model> is the symbol we passed to belongs_to.

  EXAMPLE : You want to test that a Citizenship instance refers to an instance of Country and an instance of Person; in that case, you might write the following:

  class Citizenship < ActiveRecord::Base
    belongs_to :country
    belongs_to :person
  
    validates :country, presence: true
    validates :person, presence: true
  end

ActiveRecord::Base comes with a slew of other validators we can use, as well as the mechanisms to create our own custom validators.

• Active Record Migrations - Rails Guides
• Active Record Migrations - api
• Active Record Associations - Rails Guides
• Active Record Associations - api
• Active Record Validations