Generate Models from Tables - Legacy Data Gem

November 05, 2009

Today I'd like to announce the release of a gem I've been working on Legacy Data

Getting started on a Rails project with a large existing database can be daunting. How to you extract all the information that's
encoded in the database? Do you have to understand the entire data model before you get started? The models_from_tables generator
in the legacy_data gem can help! This generator looks into your existing database and generates ActiveRecord models based on the
information encoded in it.

How to use it

  • To generate an ActiveRecord model for each table in the database just type
    script/generate models_from_tables

  • If you don't want all tables in the database tell it which table to model
    script/generate models_from_tables --table-name comments
    This uses any foreign_key constraints in the database to spider the database and model the comments table and all associated tables.

  • If you really only want the comments table tell it not to follow any foreign_keys
    script/generate models_from_tables --table-name comments --skip-associated

  • If you use factory girl (and everyone should) it will generate a simple factory for each model it generates
    script/generate models_from_tables --table-name comments --with-factories

(You do need to install the plugin gem install legacy_data as long as is one of your gem sources)


Several examples come with the gem source in the examples folder on github. These include

  • A simple blog database tested with MySQL and Sqlite3

  • The Drupal 6.14 database tested with MySQL

  • The J2EE Petstore example tested with MySQL, Sqlite3 and Oracle

What kind of information can it extract from the database?


If the database contains foreign_key constraints it uses them to build has_many or belongs_to associations
in your ActiveRecord models

Validation constraints

It will generate the following types of validation constraints in your models

  • validates_uniqueness_of - For columns where the database has an index that enforces uniqueness

  • validates_presence_of - When the database column is non-nullable

  • validates_inclusion_of - For non-nullable boolean columns and custom constraints with a SQL rule flag IN ('Y', 'N')

  • validates_numericality_of - For integer columns (nullable and non-nullable)

  • custom validation - For custom SQL validation rules in the database it puts a placeholder in your model with the original SQL for you to translate into Ruby

Non-Rails naming conventions

Since the database is existing it's likely that it doesn't follow Rails naming conventions. Not to worry as the generator will
put the non-standard name into the generated models if it needs to.

What kinds of non-standard names can it generate?

Let's look at a sample output

class Post < ActiveRecord::Base

  set_table_name  :tbpost
  set_primary_key :postid
  # Relationships
  has_many :comments, :foreign_key => :postid

  # Constraints
  validates_presence_of :title, :body

  • Class Names - It named the model Post instead of the Rails convention Tbpost. The generator could not do this itself but knowing the conventions will often not apply to legacy databases it pauses after spidering the database giving you a chance to override the table to class name mapping. It generates a yaml file app/models/table_mappings.yml where you can verify or change any class name before proceeding to generate the models.

  • Table Names - It overrode the table name since the actual name tbpost does not match the Rails naming convention posts

  • Primary Keys - It overrode the primary key since the actual column postid does not match the Rails naming convention id

  • Foreign Keys - It overrode the foreign key on the comment table to be postid instead of the Rails naming convention id