Database constraints made easy for ActiveRecord.
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Rein

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Data integrity is a good thing. Constraining the values allowed by your application at the database-level, rather than at the application-level, is a more robust way of ensuring your data stays sane.

Unfortunately, ActiveRecord doesn't encourage (or even allow) you to use database integrity without resorting to hand-crafted SQL. Rein (pronounced "rain") adds a handful of methods to your ActiveRecord migrations so that you can easily tame the data in your database.

All methods in the DSL are automatically reversible, so you can take advantage of reversible Rails migrations.

Table of Contents

Getting Started

Install the gem:

> gem install rein

Add a constraint to your migrations:

class CreateAuthorsTable < ActiveRecord::Migration
  def change
    create_table :authors do |t|
      t.string :name, null: false
    end

    # An author must have a name.
    add_presence_constraint :authors, :name
  end
end

Constraint Types

Foreign Key Constraints

A foreign key constraint specifies that the values in a column must match the values appearing in some row of another table.

For example, let's say that we want to constrain the author_id column in the books table to one of the id values in the authors table:

add_foreign_key_constraint :books, :authors

Adding a foreign key doesn't automatically create an index on the referenced column. Having an index will generally speed up any joins you perform on the foreign key. To create an index you can specify the index option:

add_foreign_key_constraint :books, :authors, index: true

Rein will automatically infer the column names for the tables, but if we need to be explicit we can using the referenced and referencing options:

add_foreign_key_constraint :books, :authors, referencing: :author_id, referenced: :id

We can also specify the behaviour when one of the referenced rows is updated or deleted:

add_foreign_key_constraint :books, :authors, on_delete: :cascade, on_update: :cascade

Here's all the options for specifying the delete/update behaviour:

  • no_action: if any referencing rows still exist when the constraint is checked, an error is raised; this is the default behavior if you do not specify anything.
  • cascade: when a referenced row is deleted, row(s) referencing it should be automatically deleted as well.
  • set_null: sets the referencing columns to be nulls when the referenced row is deleted.
  • set_default: sets the referencing columns to its default values when the referenced row is deleted.
  • restrict: prevents deletion of a referenced row.

To remove a foreign key constraint:

remove_foreign_key_constraint :books, :authors

Unique Constraints

A unique constraint specifies that certain columns in a table must be unique.

For example, all the books should have unique ISBNs:

add_unique_constraint :books, :isbn

By default, the database checks unique constraints immediately (i.e. as soon as a record is created or updated). If a record with a duplicate value exists, then the database will raise an error.

Sometimes it is necessary to wait until the end of a transaction to do the checking (e.g. maybe you want to swap the ISBNs for two books). To do so, you need to tell the database to defer checking the constraint until the end of the current transaction:

BEGIN;
SET CONSTRAINTS books_isbn_unique DEFERRED;
UPDATE books SET isbn = 'foo' WHERE id = 1;
UPDATE books SET isbn = 'bar' WHERE id = 2;
COMMIT;

This blog post offers a good explanation of how to do this in a Rails app when using the acts_as_list plugin.

If you always want to defer checking a unique constraint, then you can set the deferred option to true:

add_unique_constraint :books, :isbn, deferred: true

If you really don't want the ability to optionally defer a unique constraint in a transaction, then you can set the deferrable option to false:

add_unique_constraint :authors, :name, deferrable: false

Inclusion Constraints

An inclusion constraint specifies the possible values that a column value can take.

For example, we can ensure that state column values can only ever be available or on_loan:

add_inclusion_constraint :books, :state, in: %w[available on_loan]

To remove an inclusion constraint:

remove_inclusion_constraint :books, :state

You may also include an if option to enforce the constraint only under certain conditions, like so:

add_inclusion_constraint :books, :state,
  in: %w[available on_loan],
  if: "deleted_at IS NULL"

You may optionally provide a name option to customize the name:

add_inclusion_constraint :books, :state,
  in: %w[available on_loan],
  name: "books_state_is_valid"

Length Constraints

A length constraint specifies the range of values that the length of a string column value can take.

For example, we can ensure that the call_number can only ever be a value between 1 and 255:

add_length_constraint :books, :call_number,
  greater_than_or_equal_to: 1,
  less_than_or_equal_to: 255

Here's all the options for constraining the values:

  • equal_to
  • not_equal_to
  • less_than
  • less_than_or_equal_to
  • greater_than
  • greater_than_or_equal_to

You may also include an if option to enforce the constraint only under certain conditions, like so:

add_length_constraint :books, :call_number,
  greater_than_or_equal_to: 1,
  less_than_or_equal_to: 12,
  if: "status = 'published'"

You may optionally provide a name option to customize the name:

add_length_constraint :books, :call_number,
  greater_than_or_equal_to: 1,
  less_than_or_equal_to: 12,
  name: "books_call_number_is_valid"

To remove a length constraint:

remove_length_constraint :books, :call_number

Match Constraints

A match constraint ensures that a string column value matches (or does not match) a POSIX-style regular expression.

For example, we can ensure that the title can only contain printable ASCII characters, but not ampersands:

add_match_constraint :books, :title, accepts: '\A[ -~]*\Z', rejects: '&'

If you only want to enforce the constraint under certain conditions, you can pass an optional if option:

add_match_constraint :books, :title, accepts: '\A[ -~]*\Z', if: "status = 'published'"

You may optionally provide a name option to customize the name:

add_match_constraint :books, :title, name: "books_title_is_valid"

To remove a match constraint:

remove_match_constraint :books, :title

Numericality Constraints

A numericality constraint specifies the range of values that a numeric column value can take.

For example, we can ensure that the publication_month can only ever be a value between 1 and 12:

add_numericality_constraint :books, :publication_month,
  greater_than_or_equal_to: 1,
  less_than_or_equal_to: 12

Here's all the options for constraining the values:

  • equal_to
  • not_equal_to
  • less_than
  • less_than_or_equal_to
  • greater_than
  • greater_than_or_equal_to

You may also include an if option to enforce the constraint only under certain conditions, like so:

add_numericality_constraint :books, :publication_month,
  greater_than_or_equal_to: 1,
  less_than_or_equal_to: 12,
  if: "status = 'published'"

You may optionally provide a name option to customize the name:

add_numericality_constraint :books, :publication_month,
  greater_than_or_equal_to: 1,
  less_than_or_equal_to: 12,
  name: "books_publication_month_is_valid"

To remove a numericality constraint:

remove_numericality_constraint :books, :publication_month

Presence Constraints

A presence constraint ensures that a string column value is non-empty.

A NOT NULL constraint will be satisfied by an empty string, but sometimes you may want to ensure that there is an actual value for a string:

add_presence_constraint :books, :title

If you only want to enforce the constraint under certain conditions, you can pass an optional if option:

add_presence_constraint :books, :isbn, if: "status = 'published'"

You may optionally provide a name option to customize the name:

add_presence_constraint :books, :isbn, name: "books_isbn_is_valid"

To remove a presence constraint:

remove_presence_constraint :books, :title

Null Constraints

A null constraint ensures that a column does not contain a null value. This is the same as adding NOT NULL to a column, the difference being that it can be applied conditionally.

For example, we can add a constraint to enforce that a book has a due_date, but only if it's on_loan:

add_null_constraint :books, :due_date, if: "state = 'on_loan'"

To remove a null constraint:

remove_null_constraint :books, :due_date

Data Types

Enumerated Types

An enum is a data type that represents a static, ordered set of values.

create_enum_type :book_type, %w[paperback hardcover]

To drop an enum type from the database:

drop_enum_type :book_type

Views

A view is a named query that you can refer to just like an ordinary table. You can even create ActiveRecord models that are backed by views in your database.

For example, we can define an available_books view that returns only the books which are currently available:

create_view :available_books, "SELECT * FROM books WHERE state = 'available'"

To drop a view from the database:

drop_view :available_books

Schemas

A database can contain one or more named schemas, which in turn contain tables. Sometimes it might be helpful to split your database into multiple schemas to logically group tables together.

create_schema :archive

To drop a schema from the database:

drop_schema :archive

Examples

Let's have a look at some example migrations to constrain database values for our simple library application:

class CreateAuthorsTable < ActiveRecord::Migration
  def change
    # The authors table contains all the authors of the books in the library.
    create_table :authors do |t|
      t.string :name, null: false
      t.timestamps, null: false
    end

    # An author must have a name.
    add_presence_constraint :authors, :name
  end
end

class CreateBooksTable < ActiveRecord::Migration
  def change
    # The books table contains all the books in the library, and their state
    # (i.e. whether they are on loan or available).
    create_table :books do |t|
      t.belongs_to :author, null: false
      t.string :title, null: false
      t.string :state, null: false
      t.integer :published_year, null: false
      t.integer :published_month, null: false
      t.date :due_date
      t.timestamps, null: false
    end

    # A book should always belong to an author. The database should
    # automatically delete an author's books when we delete an author.
    add_foreign_key_constraint :books, :authors, on_delete: :cascade

    # A book must have a non-empty title.
    add_presence_constraint :books, :title

    # State is always either "available", "on_loan", or "on_hold".
    add_inclusion_constraint :books, :state, in: %w[available on_loan on_hold]

    # Our library doesn't deal in classics.
    add_numericality_constraint :books, :published_year,
      greater_than_or_equal_to: 1980

    # Month is always between 1 and 12.
    add_numericality_constraint :books, :published_month,
      greater_than_or_equal_to: 1,
      less_than_or_equal_to: 12

    # A book has a due date if it is on loan.
    add_null_constraint :books, :due_date, if: "state = 'on_loan'"
  end
end

class CreateArchivedBooksTable < ActiveRecord::Migration
  def change
    # The archive schema contains all of the archived data. We want to keep
    # this separate from the public schema.
    create_schema :archive

    # The archive.books table contains all the achived books.
    create_table "archive.books" do |t|
      t.belongs_to :author, null: false
      t.string :title, null: false
    end

    # A book should always belong to an author. The database should prevent us
    # from deleteing an author who has books.
    add_foreign_key_constraint "archive.books", :authors, on_delete: :restrict

    # A book must have a non-empty title.
    add_presence_constraint "archive.books", :title
  end
end

Contribute

PRs are always welcome! ❤️ To work with rein, there is a Makefile to keep things simple.

Before you do anything, you'll need to bootstrap your environment:

make config

Make sure you run the tests before submitting a PR:

make test

License

Rein is licensed under the MIT License.