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    Rails console

    Introducing Rails console

    Rails console is a command line program for interacting with the Rails applications. It has the full power of the Ruby language and Rails environment. We can start the Rails console using the following command:

    1bundle exec rails console

    The above command would show a console like so:


    Let's run some commands to see how Rails console can help us get some useful information.

    Finding column names of the table

    Task.column_names returns the names of all the columns of tasks table:

    1irb(main):001:0> Task.column_names
    2=> ["id", "title", "created_at", "updated_at"]

    Notice that we have a column named id present. Rails adds this column by default and it serves as the primary key of the table. The value of the id auto increments when a new record is created. We will see it in action shortly.

    Finding number of records in the table

    Task.count returns the number of records in the table:

    1irb(main):002:0> Task.count
    2   (0.1ms)  SELECT COUNT(*) FROM "tasks"
    3=> 0

    It means right now we do not have any record in the Tasks table.

    Creating a new task using Task.create

    Task.create creates and saves a new task in the database:

    1irb(main):003:0> Task.create
    2   (0.3ms)  begin transaction
    3  SQL (2.8ms)  INSERT INTO "tasks" ("created_at", "updated_at") VALUES (?, ?)  [["created_at", "2019-01-28 17:00:26.379031"], ["updated_at", "2019-01-28 17:00:26.379031"]]
    4   (11.7ms)  commit transaction
    5=> #<Task id: 1, title: nil, created_at: "2019-01-28 17:00:26", updated_at: "2019-01-28 17:00:26">

    Notice that the id is 1. If we create one more task then the id of the new task will be 2.

    We can exit the Rails console by typing exit.

    Clearing the console

    On Mac we can use Command + K. On Linux we can use Ctrl + l.

    Reloading the console

    To reload the console we can use the reload! command:

    1>> reload!
    3=> true

    Let's say that we have "rails console" open. Now if we make any changes to any model then that change would not be reflected in the console. One solution is to exit the console and start console again. The another solution is to execute reload! command.

    Getting value of the last expression

    Say we write some query into the console and forget to assign it to a variable. In the next line we want to refer to the returned value. we can use the special variable, underscore(_), to retrieve the result of the last expression.

    1> Task.find(1)
    2  Task Load (0.3ms)  SELECT "tasks".* FROM "tasks" WHERE "tasks"."id" = ? LIMIT ?  [["id", 1], ["LIMIT", 1]]
    3 => #<Task id: 1, title: "My first task", created_at: "2020-03-30 08:55:09", updated_at: "2020-04-03 09:01:42">
    4> _
    5 => #<Task id: 1, title: "My first task", created_at: "2020-03-30 08:55:09", updated_at: "2020-04-03 09:01:42">

    We can see in the above example we are using the _ to access the last expressions value.

    Use c for shortcut

    Rails console can be started by either of the following two commands:

    1bundle exec rails console
    1bundle exec rails c

    Switching the environment

    Rails console can be started in staging or in production mode like this:

    1bundle exec rails console -e production
    2bundle exec rails console -e staging

    When we do bundle exec rails console then it is equivalent to the followings:

    1bundle exec rails console -e development

    Command history

    If we want to re-execute a previously executed command then we can have two options.

    We can use the up and down arrow to get to the previously typed command.

    We can also use ctrl+r to search the previously history just like it can be done on bash shell.

    Use source location to know the location of the code

    If we want to know where a method is defined then we can use source_location to find that.

    We know that Task models have save method because of ActiveRecord. Let's find out where this save method is defined:

    1>> Task.first.method(:save).source_location
    2==> ["/Users/", 43]

    User helper

    Rails makes helper methods available to console:

    1>> helper.number_to_currency(10)
    2==> "$10.00"
    4>> helper.time_ago_in_words(
    5=> "less than a minute"

    Working in a sandbox

    When we open console and if we make any changes to the database then those changes are permanent in the database. However sometimes we want to test something and we want all those changes to rollback when we exit console. This is very often used in debugging in the production environment.

    1bundle exec rails console -e production --sandbox

    The above command will load the Rails console in sandbox mode. Example:

    1Loading production environment in sandbox
    2Any modifications you make will be rolled back on exit
    3>> Task.update_all(name: "new name")
    4>> exit

    Once we exit then all the task name we had changed would be rolled back.

    Use exit to quit console

    1irb(main):001:0> exit

    Simple prompt

    By default when console starts then prompt like this:

    1$ bundle exec rails console
    2Running via Spring preloader in process 77597
    3Loading development environment (Rails
    4irb(main):001:0> a = 1
    5irb(main):002:0> b = 2
    6irb(main):003:0> c = 3

    We can also ask console to have simple prompt:

    1$ bundle exec rails console -- --simple-prompt
    2Running via Spring preloader in process 77680
    3Loading development environment (Rails
    4>> a = 1
    5>> b = 2
    6>> c = 3

    There is nothing to commit in this chapter since all we had done was work in the Rails console.