Join the exodus and make managing your database schemas a snap. Developers are flocking to Ruby on Rails at an incredible pace.

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There's a great reason for this migration: Rails provides a robust framework built on one of the most flexible languages ever conceived. One of the tricks in the Rails bag is the concept of "Migrations. Most have relied on storing DDL in revision control, ever vigilant to ensure that our database creation scripts are updated and consistent with each rollout. That solution can be very clumsy in an Extreme Programming project.

And because Rails encourages iterative development, it would be very easy to imagine the constant schema changes turning into nightmares. Fortunately, Migrations allows a developer to manage rollout, and rollback, of database schema changes in a controlled and consistent manner, and one that happens to feel very natural to a Rails programmer.

There's probably as many ways to manage a schema for a project as there are open source content management systems. What most developers end up settling on is some sort of combination of ongoing database creation scripts and data load scripts written in pure SQL. They'll often look something like this:. There are quite a few problems with this approach, though.

While it gets the job done, it's entirely inflexible. Every time you need to change the data model for any given enhancement one or both of these files must change. That's not so much a problem if you're using a version control system like Subversion or CVS because you'll at least have a version history. But if you're working on a project for distribution where you're maintaining three or four different sets of files for each database, it can be very painful—and completely contrary to the principle of DRY.

Even with a single set of database files rolling out your changes is problematic at best. And if you need to roll the change back, it's likely that the only way to do it is to drop all the tables, restore to a known good version of your schema, and reload the data from a backup.

You could choose to try to roll back the changes by retracing your steps, but it's very error prone. Another potential problem area is portability. If your schema generation scripts were written for a specific database like MySQL and later in the development process you decide to move to Oracle, you'll have to rewrite the SQL in its entirety.

The Migration library takes the burden of having to remember the details of each database vendor's implementation off your shoulders. You'll see how this works in practice later. To get the most out of this article you'll want to have at least a rudimentary understanding of the way Rails works. Jump over to "Ruby on Rails on Oracle: A Simple Tutorial" if you're new to Rails or you just need a refresher on the conventions upon which Rails relies.

Until now, we've been captive to this ball-and-chain of SQL generation scripts or whatever your "favorite" mechanism for managing database schemas as been. Enter Rails' ActiveRecord Migrations. At the most basic level Migrations allow you to define incremental changes to your data model and data itself! This sort of approach melds seamlessly with Agile and XP methodologies favored by Rails developers the world over. A Migration is defined by subclassing ActiveRecord::Migration and overriding the two required method definitions, self.

Within the up and down definitions you can create tables and indexes, add data, manipulate models and more. The concept is simple in practice and theory: Actions you want to perform to migrate to the next version of the schema go in the up method definition. Anything you need to do to revert the change goes in the down definition. For example:. On line 3 we create the table definition and enter a block where we can alter the table by adding, dropping or otherwise modifying the table definition.

In this case we created a new column called "name" of type string. Rails maps Oracle data types to Ruby logical types like :string, :decimal, :text and so on. To revert our change is simple, you need only drop the table you created in the up definition.

On line 9 you do just that: drop the table. Couldn't be easier, right?Rails Migration uses Ruby to define changes to your database schema, making it possible to use a version control system to keep things synchronized with the actual code.

See the Rails API for details on these. A migration file contains basic Ruby syntax that describes the data structure of a database table. We will create two migrations corresponding to our three tables - books and subjects.

How to fix the Index name too long error in rails migrations

Notice that you are using lowercase for book and subject and using the plural form while creating migrations. This is a Rails paradigm that you should follow each time you create a Migration. The method self. At this moment, the above script will be used to create the books table.

The above script will be used to create subjects table; it will create five records in the subjects table. Now that you have created all the required migration files, it is time to execute them against the database.

index rails migration

Each new migration will be a new version, and any new migrations will be run, until your database is at the current version. Rake is a Ruby build program similar to Unix make program that Rails takes advantage of, to simplify the execution of complex tasks such as updating a database's structure etc.

Now we have our database and the required tables available. In the two subsequent chapters, we will explore two important components called Controller ActionController and View ActionView. Ruby on Rails 2.

index rails migration

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In the 5. Just about anything you can do with Rails 5 migrations you can now do with Standalone Migrations too! Standalone Migrations relies on the contributions of the open-source community! If you're active and do good work we'll add you as a collaborator! Install Ruby, RubyGems and a ruby-database driver e.

But if for some reason you need a specific directory structure to work with, you can use a configuration file named. These are the configurable options available.

You can omit any of the keys and Standalone Migrations will assume the default values. If you would like to use an external library such as foreigner with standalone migrations, you can add the following to your Rakefile :. Create a custom configuration file for each database and name them.

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The same conditions apply as described under Custom Configuration, however you are most likely want to specify all options to avoid conflicts and errors. If you are using Heroku or have to create or change your connection configuration based on runtime aspects maybe environment variablesyou can use the StandaloneMigrations::Configurator.

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Check the usage example:. You have to put this anywhere on your Rakefile. The on method receives the key to the configuration that you want to change within the block. The block should return your new configuration hash or nil if you want the configuration to stay the same.

Your logic to decide the new configuration need to access some data in your current configuration? Then you should receive the configuration in your block, like this:. If instead of the database-agnostic schema.This tutorial assumes you have basic Rails knowledge from reading the Getting Started with Rails Guide. There are a few commands that are absolutely critical to your everyday usage of Rails. In the order of how much you'll probably use them are:.

You can get a list of rails commands available to you, which will often depend on your current directory, by typing rails --help. Each command has a description, and should help you find the thing you need. The first thing we'll want to do is create a new Rails application by running the rails new command after installing Rails.

You can install the rails gem by typing gem install railsif you don't have it already. Rails will set you up with what seems like a huge amount of stuff for such a tiny command! You've got the entire Rails directory structure now with all the code you need to run our simple application right out of the box. The rails server command launches a web server named Puma which comes bundled with Rails.

You'll use this any time you want to access your application through a web browser. With just three commands we whipped up a Rails server listening on port The server can be run on a different port using the -p option. The default development environment can be changed using -e. The -b option binds Rails to the specified IP, by default it is localhost. You can run a server as a daemon by passing a -d option.

The rails generate command uses templates to create a whole lot of things. Running rails generate by itself gives a list of available generators:. You can install more generators through generator gems, portions of plugins you'll undoubtedly install, and you can even create your own!

Using generators will save you a large amount of time by writing boilerplate codecode that is necessary for the app to work. Let's make our own controller with the controller generator.

But what command should we use?

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Let's ask the generator:. All Rails console utilities have help text. The controller generator is expecting parameters in the form of generate controller ControllerName action1 action2.

Let's make a Greetings controller with an action of hellowhich will say something nice to us. What all did this generate? It made sure a bunch of directories were in our application, and created a controller file, a view file, a functional test file, a helper for the view, a JavaScript file, and a stylesheet file. The index parameter generates a corresponding index for the column. But instead of generating a model directly which we'll be doing laterlet's set up a scaffold.

A scaffold in Rails is a full set of model, database migration for that model, controller to manipulate it, views to view and manipulate the data, and a test suite for each of the above. We will set up a simple resource called "HighScore" that will keep track of our highest score on video games we play. Which database?

The SQLite3 database that Rails will create for you when we run the rails db:migrate command. We'll talk more about that command below. Let's talk about unit tests. Unit tests are code that tests and makes assertions about code.Migrations are a feature of Active Record that allows you to evolve your database schema over time. Migrations are a convenient way to alter your database schema over time in a consistent way.

You can think of each migration as being a new 'version' of the database.

Active Record Migrations

A schema starts off with nothing in it, and each migration modifies it to add or remove tables, columns, or entries. Active Record knows how to update your schema along this timeline, bringing it from whatever point it is in the history to the latest version. This migration adds a table called products with a string column called name and a text column called description. A primary key column called id will also be added implicitly, as it's the default primary key for all Active Record models.

These special columns are automatically managed by Active Record if they exist. Note that we define the change that we want to happen moving forward in time. Before this migration is run, there will be no table. After, the table will exist. Active Record knows how to reverse this migration as well: if we roll this migration back, it will remove the table. On databases that support transactions with statements that change the schema, migrations are wrapped in a transaction. If the database does not support this then when a migration fails the parts of it that succeeded will not be rolled back.

You will have to rollback the changes that were made by hand. There are certain queries that can't run inside a transaction. If you wish for a migration to do something that Active Record doesn't know how to reverse, you can use reversible :.

The name of the migration class CamelCased version should match the latter part of the file name. Rails uses this timestamp to determine which migration should be run and in what order, so if you're copying a migration from another application or generate a file yourself, be aware of its position in the order.

Of course, calculating timestamps is no fun, so Active Record provides a generator to handle making it for you:. This generator can do much more than append a timestamp to the file name. Based on naming conventions and additional optional arguments it can also start fleshing out the migration. If the migration name is of the form "CreateXXX" and is followed by a list of column names and types then a migration creating the table XXX with the columns listed will be generated.

For example:. As always, what has been generated for you is just a starting point. For instance:.Migrations can manage the evolution of a schema used by several physical databases. It's a solution to the common problem of adding a field to make a new feature work in your local database, but being unsure of how to push that change to other developers and to the production server. With migrations, you can describe the transformations in self-contained classes that can be checked into version control systems and executed against another database that might be one, two, or five versions behind.

This migration will add a boolean flag to the accounts table and remove it if you're backing out of the migration.

It shows how all migrations have two methods up and down that describes the transformations required to implement or remove the migration.

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See example above. Other options include :limit and :null e. Other options include :name:unique e. The null flag indicates whether the value can be NULL. Some transformations are destructive in a manner that cannot be reversed.

Migrations of that kind should raise an ActiveRecord::IrreversibleMigration exception in their down method. To run migrations against the currently configured database, use rails db:migrate. Alternatively, you can also use the STEP option if you wish to rollback last few migrations. If any of the migrations throw an ActiveRecord::IrreversibleMigration exception, that step will fail and you'll have some manual work to do.

Sometimes you'll want to add a column in a migration and populate it immediately after.

Guide to Ruby on Rails Migrations

By default, migrations will describe the actions they are taking, writing them to the console as they happen, along with benchmarks describing how long each step took. Reversible migrations are migrations that know how to go down for you. You simply supply the up logic, and the Migration system figures out how to execute the down commands for you. To define a reversible migration, define the change method in your migration like this:.

This migration will create the horses table for you on the way up, and automatically figure out how to drop the table on the way down. Some commands cannot be reversed. If you care to define how to move up and down in these cases, you should define the up and down methods as before. If a command cannot be reversed, an ActiveRecord::IrreversibleMigration exception will be raised when the migration is moving down. If the database adapter supports DDL transactions, all migrations will automatically be wrapped in a transaction.

There are queries that you can't execute inside a transaction though, and for these situations you can turn the automatic transactions off. Remember that you can still open your own transactions, even if you are in a Migration with self.

Source: show on GitHub. Disable the transaction wrapping this migration. Finds the correct table name given an Active Record object. Used to specify an operation that can be run in one direction or another. Call the methods up and down of the yielded object to run a block only in one given direction.Today, I was reviewing some legacy code, trying to isolate the bottlenecks and pinpoint the optimisation strategies for scaling and performance optimisations of a rails app.

As the usual procedure, I wanted to check the database indexes and see if something was missing here. And my guess was in the right direction. Some of the indexes were missing.

index rails migration

I thought it was gonna be a breeze, I generated the migration, filled the migration file with the correct table name and fields etc, but when I tried to run the migration, I stumbled upon an issue. After a bit of googling I learned that PostgreSQL has a limit of 63 char for naming, as can be found here. Solution : You can specify the name of the index and keep it under the 63 chars mark by passing values to the name argument in your migration.

Stay up to date! So the next obvious thing to do: Create a migration for adding the missing indexes. So I need to keep my index identified under 63 chars. Check your inbox and click the link to confirm your subscription. You've successfully subscribed to Today I learned! Subscribe to Today I learned Stay up to date!


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