What are models?
In simple terms, models are Ruby classes that can hold the value of a single row
in a database table. Since they all inherit ActiveRecord::Base through
ApplicationRecord class, they are equipped with all the ActiveRecord methods
which enables them to interact with the database.
Unlike most other frameworks, we won't be rewriting all the fields in a Rails
model definition. The fields are the column names in the mapped table. Rails
will dynamically add the fields with their value it creates an instance of the
model.
So, if there aren't any custom logic or configuration, the model definition will
be an empty class.
Creating Task model
In one of the previous chapters, we had added a migration script that creates a
new tasks table. The table contained a single column, title apart from the
special columns (created_at, updated_at).
Now we can define our Task model. Run this command in the terminal opened at
project's root folder:
The file task.rb is now created. Open file app/models/task.rb and add
following lines and save it:
We are done! We have created our first model, Task.
Model and database mapping
Let's understand the mapping of a model to the database by considering our
Task model as example:
- The model
Task in the application maps to the tasks table in database.
- The instance(object) of
Task represents a database row in the tasks
table.
- When we created
tasks table in database using migration in the previous
chapter, we had added fields like title.
- Thus the attribute
title of the Task model maps to the corresponding
title column of the tasks table in the database.
Auto generating model definitions
Actually, we don't need to manually code all these migration and Task definition
boilerplate every time we create a new model. Rails provides us with commands to
generate models and migrations in one go:
If you run this command now, it will exit without making any changes to our
files:
This is because we already have created the files. Leave it for now. We will use
this command when we generate our next model.
This command will generate model, migration, and test files for us and we
wouldn't have to write any code. Also, this will keep the linguistic convention,
that Rails uses for naming, intact. i.e. table will be named plural and the
model will be singular.
The migration generation command and model generation command have different use
cases. Migrations are atomic and thus needn't maintain one-to-one mapping with
model files. But that's not the same case when coming to models. If we are
generating a new model then we need a migration to add it to database. That's
why the latter command generates both in one go.
Now, let us commit the changes we have made before proceeding to the next
chapter.