Object Relational Mapping

An overview of Object Relational Mapping (ORM) and how is it used in Wheels. Learn how ORM simplifies your database interaction code.

Mapping objects in your application to records in your database tables is a key concept in Wheels. Let's take a look at exactly how this mapping is performed.

Class and Object Methods

Unlike most other languages, there is no notion of class level (a.k.a. "static") methods in CFML. This means that even if you call a method that does not need to use any instance data, you still have to create an object first.

In Wheels, we create an object like this:

model("author");

The built-in Wheels model() function will return a reference to an author object in the application scope (unless it's the first time you call this function, in which case it will also create and store it in the application scope).

Once you have the author object, you can start calling class methods on it, like findByKey(), for example. findByKey() returns an instance of the object with data from the database record defined by the key value that you pass.

Obviously, author is just an example here, and you'll use the names of the .cfc files you have created in the app/models folder.

authorClass = model("author");
authorObject = authorClass.findByKey(1);

For readability, this is usually combined into the following:

authorObject = model("author").findByKey(1);

Now authorObject is an instance of the Author class, and you can call object level methods on it, like update() and save().

authorObject.update(firstName="Joe");

In this case, the above code updates firstName field of the author record with a primary key value of 1 to Joe.

Primary Keys

Traditionally in Wheels, a primary key is usually named id, it increments automatically, and it's of the integer data type. However, Wheels is very flexible in this area. You can setup your primary keys in practically any way you want to. You can use natural keys (varchar, for example), composite keys (having multiple columns as primary keys), and you can name the key(s) whatever you want.

You can also choose whether the database creates the key for you (using auto-incrementation, for example) or create them yourself directly in your code.

What's best, Wheels will introspect the database to see what choices you have made and act accordingly.

Tables and Classes

Wheels comes with a custom built ORM. ORM stands for "Object-Relational Mapping" and means that tables in your relational database map to classes in your application. The records in your tables map to objects of your classes, and the columns in these tables map to properties on the objects.

To create a class in your application that maps to a table in your database, all you need to do is create a new class file in your app/models folder and make it extend the Model.cfc file.

component extends="Model" {
}

If you don't intend to create any custom methods in your class, you can actually skip this step and just call methods without having a file created. It will work just as well. As your application grows, you'll probably want to have your own methods though, so remember the app/models folder. That's where they'll go.

Once you have created the file (or deliberately chosen not to for now), you will have a bunch of methods available handle reading and writing to the authors table. (For the purpose of showing some examples, we will assume that you have created a file named Author.cfc, which will then be mapped to the authors table in the database).

For example, you can write the following code to get the author with the primary key of 1, change his first name, and save the record back to the database.

auth = model("author").findByKey(1);
auth.firstName = "Joe";
auth.save();

This code makes use of the class method findByKey(), updates the object property in memory, and then saves it back to the database using the object method save(). We'll get back to all these methods and more later.

Table and CFC Naming

By default, a table name should be the plural version of the class name. So if you have an Author.cfc class, the table name should be authors.

To change this behavior you can use the table() method. This method call should be placed in the config() method of your class file, which is where all configuration of your model is done.

So, for example, if you wanted for your author model to map to a table in your database named tbl_authors, you would add the following code to the config() method:

component extends="Model" {
    function config() {
    table("tbl_authors");
  }
}

Models Without Database Tables

Most of the time, you will want to have your model mapped to a database table. However, it is possible to skip this requirement with a simple setting:

function config() {
    table(false);
}

With that in place, you have the foundation for a model that never touches the database. When you call methods like save(), create(), update(), and delete() on a tableless model, the entire model lifecycle will still run, including object validation and object callbacks.

Typically, you will want to configure properties and validations manually for tableless models and then override save() and other persistence methods needed by your application to persist with the data elsewhere (maybe in the session scope, an external NoSQL database, or as an email sent from a contact form).

Features supported:

• Properties

• Validations

• Callbacks involving initialisation and validations

See Building search forms with tableless models in Wheels for a worked-out example.

Columns and Properties

Objects in Wheels have properties that correspond to the columns in the table that it maps to. The first time you call a method on a model, Wheels will reflect on the schema inside the database for the table the class maps to and extract all the column information.

Note about database permissions

In order for Wheels to successfully read all schema data from your database be sure the data source user has the required access for your DBMS. For example, Microsoft SQL Server requires the "ddl_admin" permission for some meta data such as column defaults.

To keep things as simple as possible, there are no getters or setters in Wheels. Instead, all the properties are made available in the this scope.

If you want to map a specific property to a column with a different name, you can override the Wheels mapping by using the property() method like this:

component extends="Model" {
    function config() {
    property(name="firstName", column="tbl_auth_f_name");
  }
}

Blank Strings and NULL Values

Since there is no concept of null / nil in CFML, Wheels will assume that when you save a blank string to the database it should be converted to NULL.

For this reason we recommend that you avoid having blank strings stored in the database (since there is no way to distinguish them from NULL values once they've been mapped to a Wheels object / result set).