SchemaAssociations

SchemaAssociations is an ActiveRecord extension that keeps your model class definitions simpler and more DRY, by automatically defining associations based on the database schema.

One of the great things about Rails (ActiveRecord, in particular) is that it inspects the database and automatically defines accessors for all your columns, keeping your model class definitions simple and DRY. That's great for simple data columns, but where it falls down is when your table contains references to other tables: then the "accessors" you need are the associations defined using belongs_to, has_one, has_many, and has_and_belongs_to_many -- and you need to put them into your model class definitions by hand. In fact, for every relation, you need to define two associations each listing its inverse, such as

class Post < ActiveRecord::Base
    has_many :comments, inverse_of: :post
end

class Comment < ActiveRecord::Base
    belongs_to :post, inverse_of: :comments
end

....which isn't so DRY.

Enter the SchemaAssociations gem. It extends ActiveRecord to automatically define the appropriate associations based on foreign key constraints in the database.

SchemaAssociations works particularly well with the schema_auto_foreign_keys gem which automatically defines foreign key constraints. So the common case is simple -- if you have this in your migration:

create_table :posts do |t|
end

create_table :comments do |t|
    t.integer post_id
end

Then all you need for your models is:

class Post < ActiveRecord::Base
end

class Comment < ActiveRecord::Base
end

and SchemaAssociations defines the appropriate associations under the hood.

You're always free to define associations yourself, if for example you want to pass special options. SchemaAssociations won't clobber any existing definitions.

You can also control the behavior with various options, via a global initializer and/or per-model. See the Configuration section for the available options.

You can globally turn off automatic creation in config/initializers/schema_associations.rb:

SchemaAssociations.setup do |config|
    config.auto_create = false
end

Then in any model where you want automatic associations, just do

class Post < ActiveRecord::Base
    schema_associations
end

You can also pass options as described in Configuration

The common cases work entirely as you'd expect. For a one-to-many relationship using standard naming conventions:

#
# migration:
#
create_table :comments do |t|
    t.integer post_id
end

#
# schema_associations defines:
#
class Post < ActiveRecord::Base
    has_many :comments
end

class Comment < ActiveReocrd::Base
    belongs_to :post
end

For a one-to-one relationship:

#
# migration:
#
create_table :comments do |t|
    t.integer post_id, index: :unique    # (using the :index option provided by schema_plus )
end

#
# schema_associations defines:
#
class Post < ActiveRecord::Base
    has_one :comment
end

class Comment < ActiveReocrd::Base
    belongs_to :post
end

And for many-to-many relationships:

#
# migration:
#
create_table :groups_members do |t|
    integer :group_id
    integer :member_id
end

#
# schema_associations defines:
#
class Group < ActiveReocrd::Base
    has_and_belongs_to_many :members
end

class Member < ActiveRecord::Base
    has_and_belongs_to_many :groups
end

Sometimes you want or need to deviate from the simple naming conventions. In this case, the belongs_to relationship name is taken from the name of the foreign key column, and the has_many or has_one is named by the referencing table, suffixed with "as" the relationship name. An example should make this clear...

Suppose your company hires interns, and each intern is assigned a manager and a mentor, who are regular employees.

create_table :interns do |t|
    t.integer :manager_id,      references: :employees
    t.integer :mentor_id,       references: :employees
end

SchemaAssociations defines a belongs_to association for each reference, named according to the column:

class Intern < ActiveRecord::Base
    belongs_to  :manager, class_name: "Employee", foreign_key: "manager_id"
    belongs_to  :mentor,  class_name: "Employee", foreign_key: "mentor_id"
end

And the corresponding has_many association each gets a suffix to indicate which one relation it refers to:

class Employee < ActiveRecord::Base
    has_many :interns_as_manager, class_name: "Intern", foreign_key: "manager_id"
    has_many :interns_as_mentor,  class_name: "Intern", foreign_key: "mentor_id"
end

If your forward relation is named "parent", SchemaAssociations names the reverse relation "child" or "children". That is, if you have:

create_table :nodes do |t|
    t.integer :parent_id         # schema_plus assumes it's a reference to this table
end

Then SchemaAssociations will define

class Node < ActiveRecord::Base
    belongs_to :parent, class_name: "Node", foreign_key: "parent_id"
    has_many :children, class_name: "Node", foreign_key: "parent_id"
end

For modularity in your tables and classes, you might use a common prefix for related objects. For example, you may have widgets each of which has a color, and each widget might have one frob that has a top color and a bottom color--all from the same set of colors.

create_table :widget_colors do |t|
end

create_table :widgets do |t|
    t.integer   :widget_color_id
end

create_table :widget_frobs do |t|
    t.integer :widget_id, index: :unique
    t.integer :top_widget_color_id,    references: :widget_colors
    t.integer :bottom_widget_color_id, references: :widget_colors
end

Using the full name for the associations would make your code verbose and not quite DRY:

@widget.widget_color
@widget.widget_frob.top_widget_color

Instead, by default, SchemaAssociations uses concise names: shared leading words are removed from the association name. So instead of the above, your code looks like:

@widget.color
@widget.frob.top_color

i.e. these associations would be defined:

class WidgetColor < ActiveRecord::Base
    has_many :widgets,         class_name: "Widget",     foreign_key: "widget_color_id"
    has_many :frobs_as_top,    class_name: "WidgetFrob", foreign_key: "top_widget_color_id"
    has_many :frobs_as_bottom, class_name: "WidgetFrob", foreign_key: "bottom_widget_color_id"
end

class Widget < ActiveRecord::Base
    belongs_to :color, class_name: "WidgetColor", foreign_key: "widget_color_id"
    has_one    :frob,  class_name: "WidgetFrob",  foreign_key: "widget_frob_id"
end

class WidgetFrob < ActiveRecord::Base
    belongs_to :top_color,    class_name: "WidgetColor", foreign_key: "top_widget_color_id"
    belongs_to :bottom_color, class_name: "WidgetColor", foreign_key: "bottom_widget_color_id"
    belongs_to :widget,       class_name: "Widget",      foreign_key: "widget_id"
end

If you like the formality of using full names for the asociations, you can turn off concise names globally or per-model, see Configuration.

If the target of a has_many association has a column named position, SchemaAssociations will specify order: :position for the association. That is,

create_table :comments do |t|
    t.integer post_id
    t.integer position
end

leads to

class Post < ActiveRecord::Base
    has_many :comments, order: :position
end

SchemaAssociations determines the model class name from the table name using the same convention (and helpers) that ActiveRecord uses. But sometimes you might be doing things differently. For example, in an engine you might have a prefix that goes in front of all table names, and the models might all be namespaced in a module.

To that end, SchemaAssociations lets you configure mappings from a table name prefix to a model class name prefix to use instead. For example, suppose your database had tables:

hpy_campers
hpy_go_lucky

The default model class names would be

HpyCampers
HpyGoLucky

But if instead you wanted

Happy::Campers
Happy::GoLucky

you would define the mapping in the configuration:

SchemaPlus.setup do |config|
    config.table_prefix_map["hpy_"] = "Happy::"
end

Tables names that don't start with hpy_ will continue to use the default determination.

You can set up multiple mappings. E.g. if you're using several engines they can each set up the mapping for their own modules.

You can set up a mapping from or to the empty string, in order to unconditionally add or remove prefixes from all model class names.

If you're curious (or dubious) about what associations SchemaAssociations defines, you can check the log file. For every assocation that SchemaAssociations defines, it generates a debug entry such as

[schema_associations] Post.has_many :comments, :class_name "Comment", :foreign_key "comment_id"

which shows the exact method definition call.

SchemaAssociations defines the associations lazily, only creating them when they're first needed. So you may need to search through the log file to find them all (and some may not be defined at all if they were never needed for the use cases that you logged).

You can configure options globally in an initializer such as config/initializers/schema_associations.rb, e.g.

SchemaAssociations.setup do |config|
  config.concise_names = false
end

and/or override the options per-model, e.g.:

class MyModel < ActiveRecord::Base
  schema_associations.config concise_names: false
end

Here's the full list of options, with their default values:

SchemaAssociations.setup do |config|

  # Enable/disable SchemaAssociations' automatic behavior
  config.auto_create = true
  
  # Whether to use concise naming (strip out common prefixes from class names)
  config.concise_names = true
  
  # List of association names to exclude from automatic creation.
  # Value is a single name, an array of names, or nil.
  config.except = nil
  
  # List of association names to include in automatic creation.
  # Value is a single name, and array of names, or nil.
  config.only = nil

  # List of association types to exclude from automatic creation.
  # Value is one or an array of :belongs_to, :has_many, :has_one, and/or
  # :has_and_belongs_to_many, or nil.
  config.except_type = nil

  # List of association types to include in automatic creation.
  # Value is one or an array of :belongs_to, :has_many, :has_one, and/or
  # :has_and_belongs_to_many, or nil.
  config.only_type = nil

  # Hash whose keys are possible matches at the start of table names, and
  # whose corresponding values are the prefix to use in front of class
  # names.
  config.table_prefix_map = {}
end

SchemaAssociations is tested on all combinations of:

• ruby 2.5 with activerecord 5.2, using mysql2, postgresql:9.6 or sqlite3
• ruby 2.5 with activerecord 6.0, using mysql2, postgresql:9.6 or sqlite3
• ruby 2.5 with activerecord 6.1, using mysql2, postgresql:9.6 or sqlite3
• ruby 2.7 with activerecord 5.2, using mysql2, postgresql:9.6 or sqlite3
• ruby 2.7 with activerecord 6.0, using mysql2, postgresql:9.6 or sqlite3
• ruby 2.7 with activerecord 6.1, using mysql2, postgresql:9.6 or sqlite3
• ruby 2.7 with activerecord 7.0, using mysql2, postgresql:9.6 or sqlite3
• ruby 3.0 with activerecord 6.0, using mysql2, postgresql:9.6 or sqlite3
• ruby 3.0 with activerecord 6.1, using mysql2, postgresql:9.6 or sqlite3
• ruby 3.0 with activerecord 7.0, using mysql2, postgresql:9.6 or sqlite3
• ruby 3.1 with activerecord 6.0, using mysql2, postgresql:9.6 or sqlite3
• ruby 3.1 with activerecord 6.1, using mysql2, postgresql:9.6 or sqlite3
• ruby 3.1 with activerecord 7.0, using mysql2, postgresql:9.6 or sqlite3

Notes:

• As for version 1.3.0, rails < 5.2 and ruby < 2.5 are no longer supported
• As of version 1.2.3, rails < 4.1 and ruby < 2.1 are no longer supported
• As of version 1.2.0, ruby 1.9.2 is no longer supported.
• As of version 1.0.0, ruby 1.8.7 and rails < 3.2 are no longer supported.

Install from http://rubygems.org via

$ gem install "schema_associations"

or in a Gemfile

gem "schema_associations"

• Add AR 6.1 and 7.0
• Add Ruby 3.1
• drop schema_plus_compatibiltiy dependency (indirect through schema_plus_foreign_keys update)

• add AR 6.0
• add Ruby 3.0
• drop AR < 5.2
• drop Ruby < 2.5

• add in auto deferring of has_* :through associations manually defined on the model so they work in AR 5.1+

• Support for AR5 (Rails 5).

• Use schema_monkey rather than Railties.

• Bug fix: Don't fail trying to do associations for abstract classes (mysql2 only). [#11, #12] Thanks to @dmeranda

• Use schema_plus_foreign_keys rather than all of schema_plus, to eliminate unneeded dependancies. That limits us to AR >= 4.1 and ruby >= 2.1
• Fix deprecations
• Logging is now at debug level rather than info level

• Bug fix (Rails workaround) for STI: propagate associations to subclasses, since Rails might not, depending on the load order.

• Works with Rails 4.1
• Test against MRI ruby 2.1.2

• Works with Rails 4, thanks to @tovodeverett
• Test against MRI ruby 2.0.0; no longer test against 1.9.2

• New feature: config.table_prefix_map

• Bug fix: use singular :inverse_of for :belongs_to of a :has_one

• Use :inverse_of in generated associations

• Drop support for ruby 1.8.7 and rails < 3.2

• SchemaAssociations is derived from the "Red Hill On Rails" plugin foreign_key_associations originally created by harukizaemon (https://github.com/harukizaemon)

• SchemaAssociations was created in 2011 by Michal Lomnicki and Ronen Barzel

This gem is released under the MIT license.

Are you interested in contributing to SchemaPlus::Views? Thanks! Please follow the standard protocol: fork, feature branch, develop, push, and issue pull request.

Some things to know about to help you develop and test:

• schema_dev: SchemaAssociations uses schema_dev to facilitate running rspec tests on the matrix of ruby, activerecord, and database versions that the gem supports, both locally and on github actions

  To to run rspec locally on the full matrix, do:

    $ schema_dev bundle install
    $ schema_dev rspec
  

  You can also run on just one configuration at a time; For info, see schema_dev --help or the schema_dev README.

  The matrix of configurations is specified in schema_dev.yml in the project root.