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Derived Attributes

Automatic calculation of dependent values using OCL

Overview

A derived attribute is a value calculated from other values, eliminating the need for persistence since it can always be recomputed. Common examples include invoice totals, full names from first/last name, or age calculated from birth date.

This example demonstrates code-free derived attributes using OCL expressions defined directly in the model.

Key Concepts

Concept Description
OCL Expressions Define derivation logic in the model using Object Constraint Language
Automatic Updates Derived values recalculate automatically when dependencies change
Write Protection Derived attributes are read-only - change the source values instead
OCL Variables Use global variables within OCL expressions for dynamic calculations

How It Works

Derived attributes automatically recalculate when their dependencies change:

flowchart LR
    net["net_Price: Currency"] --> expr["net_Price * (1 + VAT)"]
    vat["VAT = 0.25"] --> expr
    expr --> val["$125.00"]

    style net fill:#3b82f6,color:#fff
    style vat fill:#f59e0b,color:#fff
    style expr fill:#334155,stroke:#3b82f6,color:#fff
    style val fill:#22c55e,color:#fff

Example Calculation: $100 × (1 + 0.25) = $125.00

How to Run the Example

  1. Add products using the grid and enter net prices
  2. Observe the Retail Price column - it's calculated automatically
  3. Change the VAT value at the bottom - watch retail prices update
  4. Try editing the Retail Price directly - it's write-protected
  5. Change Net Price - the Retail Price updates automatically

Implementation

The retail_Price attribute in the Product class is derived using an OCL expression that references the net_Price attribute and a global VAT variable.

This approach requires zero code - the derivation logic is entirely defined in the model, making it easy to modify and maintain.

Step-by-Step: Adding a Derived Attribute

This workflow demonstrates adding a ProjectDuration attribute to a Process class that calculates days between StartDate and EndDate.

Video Walkthrough

Complete workflow: Adding a derived attribute in the Model Editor

1. Open the Model Editor

Double-click on the BoldModel1 component in your form or datamodule. The Bold Model Editor opens.

2. Navigate to the Class

In the class tree, find and select the target class (e.g., Process).

3. Add New Attribute

Right-click on the class attributes section and select Add Attribute, or use the toolbar button.

4. Configure the Attribute Properties

Property Value Description
Name ProjectDuration Attribute name
Type Integer Return type of the calculation
Derived True Mark as derived (calculated)
Persistent False Not stored in database
DelayedDerive False Recalculate immediately when dependencies change

5. Set the DerivationOCL Expression

Enter the OCL expression that calculates the value:

if (self.StartDate.isNull or self.EndDate.isNull) then nil else (self.EndDate - self.StartDate).round endif

OCL Expression Explained

  • self.StartDate.isNull - Check if StartDate has no value
  • self.EndDate - self.StartDate - Date subtraction returns days as float
  • .round - Convert float to integer (rounds to nearest)
  • nil - Return null when dates are missing

6. Save and Regenerate Code

Save the model and regenerate BusinessClasses.pas. The derived attribute is now ready to use.

Behavior

When StartDate or EndDate changes, ProjectDuration automatically recalculates. UI components bound to it update immediately.

With Persistent=False, the value is not stored. It's calculated on demand when the object is loaded. For persistent derived attributes, set Persistent=True.

Project Files

:material-file-code: Location: examples/Simple/ObjectSpace/DerivedAttributes/

  • BE_DerivedAttribDemo.dpr - Main project file