Dictionary Overrides

Dictionary Overrides in ServiceNow – A Complete Guide for CAD Exam Preparation

Introduction

Dictionary Overrides are a fundamental concept in ServiceNow application design and development. Understanding how they work is critical for anyone preparing for the ServiceNow Certified Application Developer (CAD) exam, as they frequently appear in exam questions related to table inheritance and field customization.

Why Are Dictionary Overrides Important?

In ServiceNow, tables often extend (inherit from) other tables. For example, the Incident table extends the Task table. When a child table inherits fields from a parent table, those fields carry over all of their attributes — such as default values, mandatory settings, read-only status, and reference qualifiers. However, there are many situations where the behavior of an inherited field needs to differ on the child table without affecting the parent table or other child tables. This is precisely where Dictionary Overrides come in.

Without Dictionary Overrides, administrators and developers would have to either modify the parent table's field (affecting all child tables) or resort to workarounds like client scripts or business rules to enforce different behavior — both of which are less efficient and harder to maintain.

What Is a Dictionary Override?

A Dictionary Override is a mechanism in ServiceNow that allows you to change specific attributes of a field on a child table that was inherited from a parent table, without modifying the original field definition on the parent table.

In simpler terms, it lets you override certain properties of an inherited field at the child table level.

For example, the Assignment Group field exists on the Task table. If you want the Incident table to have a different default value for Assignment Group than what is set on the Task table, you would create a Dictionary Override on the Incident table for that field.

What Attributes Can Be Overridden?

Not all field attributes can be overridden using Dictionary Overrides. The key attributes that can be overridden include:

• Default value – Set a different default value for the field on the child table.
• Mandatory – Make a field mandatory on the child table even if it is not mandatory on the parent table (or vice versa).
• Read-only – Make a field read-only on the child table.
• Reference qualifier – Change the reference qualifier to filter reference field options differently on the child table.
• Dependent field – Change the dependent field relationship.
• Calculated value – Override the calculated value setting.
• Attributes – Override the field's attributes (such as edge_encryption_enabled, etc.).

Attributes that cannot be overridden include:

• Column name – The internal field name remains the same.
• Field type – You cannot change a String field to an Integer field, for example.
• Column length (max length) – The maximum length of the field cannot be changed via Dictionary Override.
• Label – The field label cannot be overridden through a Dictionary Override (labels are managed through Label entries or field labels).

Note: This distinction between what can and cannot be overridden is a very common exam topic.

How Do Dictionary Overrides Work?

Here is a step-by-step explanation of how Dictionary Overrides function:

1. Table Inheritance
When a child table extends a parent table, all fields defined on the parent are automatically available on the child table. These inherited fields share the same dictionary entry as the parent.

2. Creating a Dictionary Override
To override a field attribute on the child table, navigate to:

System Definition → Dictionary Overrides

Or, you can access it by opening the field's dictionary entry on the child table. When you modify an overridable attribute on the child table, ServiceNow automatically creates a Dictionary Override record.

3. Override Record
The Dictionary Override record specifies:
• The table where the override applies (the child table).
• The field name being overridden.
• The attribute(s) being changed and their new values.

4. Runtime Behavior
When a user accesses the child table, ServiceNow checks for any Dictionary Override records. If one exists for a given field, the overridden attributes are applied instead of the parent table's attributes. The parent table and other child tables remain completely unaffected.

Practical Example

Consider this scenario:

• The Task table has a field called Priority with a default value of 4 - Low.
• The Incident table extends Task.
• You want Incidents to have a default Priority of 3 - Moderate.

To accomplish this:
1. Navigate to System Definition → Dictionary Overrides.
2. Create a new record.
3. Set the Table to Incident [incident].
4. Set the Element to priority.
5. Set the Default value to 3.
6. Save the record.

Now, when a new Incident is created, the Priority field defaults to 3 - Moderate, while new Task records (and other child tables like Change Request or Problem) still default to 4 - Low.

Dictionary Overrides vs. Other Customization Methods

It is important to understand when to use Dictionary Overrides versus other approaches:

• Dictionary Overrides vs. Client Scripts: Dictionary Overrides are server-side and declarative — they are the preferred method for changing default values, mandatory settings, and reference qualifiers on inherited fields. Client Scripts are client-side and should be used for UI-level logic, not for basic attribute changes.

• Dictionary Overrides vs. Business Rules: While Business Rules can enforce mandatory fields or set default values, Dictionary Overrides are more efficient and maintainable for overriding inherited field properties.

• Dictionary Overrides vs. Modifying the Parent Table: Modifying the parent table affects ALL child tables. Dictionary Overrides provide granular, table-specific customization.

Key Points to Remember for the Exam

1. Dictionary Overrides only apply to inherited fields on child tables. They do not apply to fields defined directly on a table.
2. You can override: default value, mandatory, read-only, reference qualifier, calculated, dependent field, and attributes.
3. You cannot override: field type, column name, max length, or label.
4. Dictionary Overrides are defined in the sys_dictionary_override table (sys_dictionary_override.list).
5. They are a best practice approach for customizing inherited field behavior without impacting the parent or sibling tables.
6. A Dictionary Override is automatically created when you modify an overridable attribute on a child table through the form designer or dictionary entry.

Exam Tips: Answering Questions on Dictionary Overrides

Here are targeted strategies for answering CAD exam questions on this topic:

• Identify the keyword "inherited field": If a question mentions changing behavior of a field that comes from a parent table, Dictionary Override is almost always the correct answer.

• Watch for "without affecting the parent table": This phrase is a strong indicator that the answer involves a Dictionary Override.

• Know the list of overridable vs. non-overridable attributes: Many exam questions test whether you know that you cannot override field type or max length but can override default value, mandatory, and reference qualifier.

• Eliminate incorrect options: If an answer choice suggests modifying the parent table dictionary entry to change behavior on only one child table, that is incorrect. Similarly, if an answer suggests using a client script to change a default value on an inherited field, the Dictionary Override option is the better (and more correct) answer.

• Understand the scenario-based format: Exam questions often present a scenario like: "An administrator wants the State field on the Change Request table to be mandatory, but it is not mandatory on the Task table. What should they do?" The correct answer would be to create a Dictionary Override on the Change Request table for the State field and set it to mandatory.

• Remember the navigation path: Some questions may ask where Dictionary Overrides are configured. The answer is System Definition → Dictionary Overrides.

• Don't confuse Dictionary Overrides with Dictionary Entries: A Dictionary Entry (sys_dictionary) defines the field itself. A Dictionary Override (sys_dictionary_override) modifies specific attributes of that field on a child table. These are two different things.

• Reference Qualifier overrides are common in scenarios: Questions may describe a situation where a reference field on a child table needs to show a filtered set of records different from the parent table's filter. The correct approach is a Dictionary Override to change the reference qualifier.

Summary

Dictionary Overrides are a powerful and elegant feature in ServiceNow that support the platform's table inheritance model. They allow developers to customize inherited field behavior at the child table level without disrupting the parent table or other child tables. For the CAD exam, focus on understanding what can and cannot be overridden, when to use Dictionary Overrides versus other methods, and how to recognize scenarios that call for their use. Mastering this concept will help you confidently answer multiple exam questions and apply best practices in real-world ServiceNow development.