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:
The above command would show a console like this.
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:
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:
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:
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 or an object
To reload the console we can use the reload! command:
To reload the object we can use the reload method on an object:
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
reload the object with reload method on an object, like we had shown in
above code block or reload the application itself by executing reload!
command. If none of those works in reloading the latest values from the
database, then the final solution is to exit the console and start the console
again from the scratch.
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.
We can see in the above example we are using the _ to access the last
expressions value.
Group statements in ActiveRecord
In Rails, group statements enable you to perform grouping and aggregation operations on data fetched from the database. Group statements allow you to mimic the behavior of the SQL GROUP BY clause, providing a convenient way to summarize and analyze data based on specific attributes or columns.
Let's try to group the tasks by the hour of the created_at timestamp to see the number of tasks created in each hour.
We can see that it returns a hash where the keys represent the dates extracted from the created_at datetime attribute, and the values represent the number of records that have the same date. As there is currently only one instance of the Task model, the count is one. However, creating and saving more tasks will demonstrate the full usefulness of group statements. You can learn more about group statements in the official docs.
Use c as a shortcut
Rails console can be started by either of the following two commands:
Switching the environment
Rails console can be started in staging or in test environment like this:
When we do bundle exec rails console then it is equivalent to the followings:
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:
User helper
Rails makes helper methods available to console:
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 development and test environment.
The above command will load the Rails console with development environment in
sandbox mode. Example:
Once we exit then all the task name we had changed would be rolled back.
Use exit to quit console
Simple prompt
By default when console starts then prompt like this:
We can also ask console to have simple prompt:
We haven't made any changes in our project. So there is nothing to commit in this chapter.