How to mock formulas and read-only relationships in Apex tests?

Learn how to use dependency injection to mock read-only fields, such as formula fields and child relationships in your tests.

⚙️ Repository on Github: MockFieldReader.

A while ago we explored the Mock Data Layer Pattern, which allows us to mock virtually any relationships between records within your tests. This is especially handy when trying to improve the performance of these tests – more details here on matheus.dev.

Now, how can we mock formula fields and non-writable child relationships if they are, well, non-writable fields? 🤔

Mocking values in read-only fields

For this exploratory project, we will continue using the convenience of interfaces and dependency injection, but this time to develop a class used to mock field values. 

We will use a not-so-realistic scenario with parent and child accounts, but the intent is to create a test for the Account Trigger Handler that will use a Mock Field Reader and a Mock Data Layer.

The Mock Field Reader will be used so we can mock two non-writable fields on the Account object.

• Child_Accounts__r, a non-writable child relationship on the parent record
• External_URL__c, a formula field on the child record

The trigger handler logic will update the following field:

• Parent.Latest_Child_URL__c

Based on the following formula field:

• Child.External_URL__c

Files used in this project

Show me the code!

First, let’s create an interface with the required methods for the Field Reader:

Using this interface, let’s create a class that reads values from a SObject record, using standard SObject methods:

Also, using this interface, let’s create a class that will allow us to mock the non-writable fields, like a child relationship or formula fields:

Getters

To respect the injected dependency, these methods must be present on both the real instance of the field reader (FieldReader) and the mock instance of the field reader (MockFieldReader).

public Object getFieldValue(SObject record, String fieldName)

This method is used to retrieve the mock values of formula fields. It returns an Object that can later be cast as different types, such as String, Number, Id, etc.

public SObject getFieldRecord(SObject record, String fieldName)

This method is used to retrieve the mock values that represent a single record. It returns an SObject.

public List getFieldRecords(SObject record, String fieldName)

This method is used to retrieve the mock values that represent a list of records, such as a child relationship. It returns a list of SObjects.

Setter

There’s only one method used to set the values, and it’s only present on the mock instance of the field reader (MockFieldReader).

public void addValueToField(SObject record, String field, Object value)

This method adds the field/value pair to a map of mocked values, indexed by the record Id.

Examples

In the repository on Github you will find a trigger handler class and its respective test class with the usage of this Mock Field Reader.

Snippet from the AccountTriggerHandler: pay attention to how the following methods are being used:

fieldReader.getFieldRecords(parentAndChildAccount, 'Child_Accounts__r');
fieldReader.getFieldValue(latestChild, 'External_URL__c');

And in the AccountTriggerHandlerTest you can see how these non-writable fields can be mocked, using the method addValueToField:

With this code and the Mock Field Reader, it becomes easy to mock non-writable fields used in the tests.

I hope that helps!