'Connect' a rails app to an already existing MySQL DB?

Question

So in my company we are slowly moving to Rails instead of PHP(Code Igniter to be precise). So, our actual PHP App is using a Mysql DB and I'd like to connect a new Rails app to this DB but meanwhile our PHP is still running, so I can't change the DB.

I don't really know where I should start to use all the rails features (Or at least as much as possible).

php app and rails app will use the same db? or rails app has another db too to work with? — rmagnum2002, Jun 25 '13 at 13:58
then you just use you database.yml to point to the db that php uses and that's it — rmagnum2002, Jun 25 '13 at 14:04

score 3 · Accepted Answer · answered Jun 25 '13 at 13:59

3

There shouldn't be any harm in connecting your rails app to an existing database. You will need to watch for anything that goes against rails conventions (table names are plurals of models, for example) and either change the database (and your php app) or program around the problem in rails.

But the first step is simply to connect to the database and make models for the existing tables and see what works and what doesn't.

After that, post here with any specific problems.

As a suggestion, take a backup of your database and start out programming against that to build your application and be sure everything works safely.

answered Jun 25 '13 at 13:59

Matt

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Thank you! What if the table names are not complying with the rails convention ? I can't touch the DB yet, I'll need to 'hack' with rails first. – bl0b Jun 25 '13 at 14:23
Does it mean there is no better way than to generate models: table by table manually? .NET have one command only I think. – Katarzyna May 02 '15 at 21:53
Finally now in 2022 there is a good way to make this happen! A gem called [The Brick](https://github.com/lorint/brick) can auto-create migration files with `bin/rails g brick:migrations`, and models with `bin/rails g brick:models`. – Lorin Thwaits Aug 15 '22 at 02:32

score 2 · Answer 2 · answered Jun 25 '13 at 14:01

Well, first of all you should setup the connection in config/database.yml and then start to generate the scaffolding (models, views and controllers) table by table (check the Rails generate command). I am not really sure if you have already generated the app though. Anyway, the generator will also generate a migration script that you obviously dont want to run as the db is already there.

Hope this helps a bit.

Anyway, some resources:

http://guides.rubyonrails.org/
http://railsapps.github.io/

Thanks for the answer. I haven't generated the app yet. I was waiting for some clarification before starting. — bl0b, Jun 25 '13 at 14:22

score 0 · Answer 3 · answered Aug 06 '18 at 22:09

There are two aspects of a Rails app to consider for this scenario:

1: the database connection

Simply put the credentials for this database into database.yml.

A model like "User" will by default attempt to find records and attribute definitions in a table called "users". ActiveRecord will assume there's an auto-incrementing integer primary key on each table. When saving records, it will attempt to write to columns called created_at and updated_at. Those are a few things to be mindful of when making and using the connection.

2: the database migrations

Rails uses migration files to manage a sequence of changes to the database structure. Under normal conditions, someone building a Rails app will be starting with an empty database.

In the case of an existing database, I would recommend making a migration something like:

class BuildLegacyDbStructure < ActiveRecord::Migration  
  def up
    Mysql2.connection.execute_some_sql_file(  # made-up function
      Rails.root.join('path', 'to', 'file')
    )  
  end

  def down
    # reverse those changes; bring DB down to blank state
  end
end

Another option would be to disable Rails/ActiveRecord's migration-based management of the database entirely. For example Rails will generate a migration when you generate a new model. So if you have an existing users table in your PHP app, and you'd like to make a rails model to use this table, you'd run something like rails generate model User --no-migration.

Wanted to mention that [The Brick](https://github.com/lorint/brick) gem can auto-create migrations for existing tables with `bin/rails g brick:migrations`, and create models with full belongs_to / has_many support using `bin/rails g brick:models`. — Lorin Thwaits, Aug 15 '22 at 02:33

'Connect' a rails app to an already existing MySQL DB?

3 Answers3

1: the database connection

2: the database migrations

Linked