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 ColdFusion. 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:
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
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.
author is just an example here, and you'll use the names of the .cfc files you have created in the models folder.
<cfset authorClass = model("author")>
<cfset authorObject = authorClass.findByKey(1)>
For readability, this is usually combined into the following:
<cfset authorObject = model("author").findByKey(1)>
In this case, the above code updates
firstName field of the
author record with a primary key value of
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 models folder and make it extend the Model.cfc file.
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 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.
<cfset auth = model("author").findByKey(1)>
<cfset auth.firstName = "Joe">
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
init() method of your class file.
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
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 (or every time if you're in
design mode), Wheels will reflect on the schema inside the database for the table the class maps to and extract all the column information.
To keep things as simple as possible, there are no getters or setters in Wheels. Instead, all the properties are made available in the
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:
<cfset property(name="firstName", column="tbl_auth_f_name")>