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Creating Tables | Activity Tracker

We will learn about creating tables in Rails while keeping ourselves fit!

Tables are the database objects that contain all the data in a database. In tables, data is locally organized in a row-and-column format similar to a spreadsheet. Each row represents a unique record and each column represents a field in the record.

For example, a table that contains employee data might contain a row for each employee and columns representing employee information such as employee number, name, address, job title and phone number.

Tables are created executing the SQL statement CREATE TABLE on the database. For example:

CREATE TABLE employees (
  employee_number INTEGER,
  name            VARCHAR(80),
  address         VARCHAR(255),
  title           VARCHAR(80),
  phone_number    VARCHAR(10)
  salary          DECIMAL
);

In Rails, each table represents a class in app/models and creating new tables is simpler and more convenient, thanks to the generator and migrations.

Rails equivalent of the above statement would be:

rails generate model Employee employee_number:integer name:string \
address:string title:string phone_number:string{10} salary:decimal

For this exercise, we will create a table activity with the following attributes and data-types:

  • Title: string
  • Type: string
  • Start: datetime
  • Duration: decimal
  • Calories: integer

Steps

  1. Prepare Local Database

  • Change directory to activity-tracker

    cd activity-tracker

  • Create the database

    rails db:create

    You should see two new files are created if not present already - development.sqlite3 and test.sqlite3.

  • Run the migrations

    rails db:migrate

  1. Create the tables

    Models can be created using Rails generator with the following syntax:

    rails generate model NAME field[:type] field[:type]...

    So for our requirement, execute the following command:

    rails generate model Activity title:string type:string start:datetime duration:decimal calories:integer

    Rails has great emphasis on naming conventions. The model name must be CamelCase and column names must be snake_case. Read about other rails naming conventions.

    The command creates multiple files - most important of which are app/models/activity.rb and db/migrate/<YYYYMMDDHHMMSS>_create_activities.rb

    The number in the beginning of the migration file is a UTC timestamp. Rails uses this timestamp to determine which migration should be run and in what order.

    The file db/migrations/<YYYYMMDDHHMMSS>_create_activities.rb looks like:

    class CreateActivities < ActiveRecord::Migration[7.0]
def change
    create_table :activities do |t|
      t.string :title
      t.string :type
      t.datetime :start
      t.decimal :duration
      t.integer :calories

      t.timestamps
    end
  end
end

    The migration roughly translates to "create table activities with the five columns and their types described above" which is exactly what we wanted! The migration also uses the code t.timestamps to add timestamps created_at and updated_at to the activities table automatically.

    • Run the migration.
    rails db:migrate

    Running the migration commits the changes to the database. Therefore, your changes are not reflected in the database unless migrated.

    • Execute the test suite and submit your changes when tests pass.
    rails test

Rolling Back Migrations

If the migration was incorrect (say wrong column names or datatype), you can "undo" the migration by the following command:

rails db:rollback

Then, fix the migration file db/migrations/<YYYYMMDDHHMMSS>_create_students.rb and re-run the migration to commit your changes to the dabase.

Read more about migrations at: Running Migrations | Active Record Migrations


References