description: >- Generate extra properties in your models on the fly without needing to store redundant data in your database.

Calculated Properties

Working within CFML's Constraints to Deliver OOP-like Functionality

Wheels makes up for the slowness of arrays of objects in CFML by providing calculated properties. With calculated properties, you can generate additional properties on the fly based on logic and data within your database.

Example #1: Full Name

Consider the example of fullName. If your database table has fields for firstName and lastName, it wouldn't make sense to store a third column called fullName. This would require more storage for redundant data, and it would add extra complexity that could lead to bugs and maintenance problems in the future.

Traditional Object-Oriented Calculations

In most object-oriented languages, you would add a method to your class called getFullName(), which would return the concatenation of this.firstName & " " & this.lastName. The getFullName() method could potentially provide arguments to list the last name first and other types of calculations or transformations as well.

Wheels still allows for you to do this sort of dynamic calculation with the returnAs="objects" argument in methods like findAll(), but we advise against it when fetching large data sets because of the slowness of CreateObject() across CFML engines.

See the chapter on Reading Records for more information.

Using Calculated Properties to Generate fullName in the Database at Runtime

As an alternative, you can set up a calculated property that dynamically performs the concatenation at the database level. In our example, we would write a line similar to this in our model's config() method:

property(
        name="fullName",
        sql="RTRIM(LTRIM(ISNULL(users.firstname, '') + ' '
            + ISNULL(users.lastname, '')))"
    );

As you can probably deduce, we're creating a SQL statement that will be run in the SELECT clause to generate the `fullName.

With this line in place, fullName will become available in both full model objects and query objects returned by the various finder methods like findAll() and findOne().

Example #2: Age

Naturally, if you store the user's birth date in the database, your application can use that data to dynamically calculate the user's age. Your app always knows how many years old the user is without needing to explicitly store his or her age.

Creating the Calculated Property for Age

In order to calculate an extra property called age based on the birthDate column, our calculated property in config() may look something like this:

Example Code

property(
        name="age",
        sql="(CAST(CONVERT(CHAR(8), GETDATE(), 112) AS INT)
            - CAST(CONVERT(CHAR(8), users.date_of_birth, 112) AS INT))
            / 10000"
);

Much like the fullName example above, this will cause the database to add a property called age storing the user's age as an integer.

Note that the cost to this approach is that you may need to introduce DBMS-specific code into your models. This may cause problems when you need to switch DBMS platforms, but at least all of this logic is isolated into your model CFCs.

Using the New age Property for Other Database Calculations

Calculated properties don't end at just generating extra properties. You can now also use the new property for additional calculations:

• Creating additional properties with the select argument
• Additional where clause calculations
• Record sorting with order
• Pagination
• And so on…

For example, let's say that we only want to use age to return users who are in their 20s. We can use the new ageproperty as if it existed in the database table. For extra measure, let's also sort the results from oldest to youngest.

Example Code

users = model("user").findAll(
        where="age >= 20 AND age < 30", order="age DESC"
);

Specifying a Data Type

By default, calculated properties will return char as the column data type. Whilst this covers most scenarios, if you want to return something like a date, it can be problematic. Thankfully we can just specify a dataType argument to return the appropriate data type.

property(
  name="createdAtAlias",
  sql="posts.createdat", 
  dataType="datetime"
);