Child Relationships are mostly automatic. The work is naming them well and using them correctly in code and reports.
- Set Child Relationship Names at field creation
When creating a lookup or master-detail field, set the Child Relationship Name to something readable. Defaults work but customisation pays off for life.
- Use SOQL subqueries to walk children
For parent-to-child queries, use the subquery syntax: SELECT Id, (SELECT Id FROM Cases) FROM Account. The relationship name in parentheses is the Child Relationship Name.
- Access children in Apex through the relationship name
account.Cases returns the list of Cases from a subquery. The name matches the Child Relationship Name set on the field.
- Inspect relationships through Schema Builder
When unsure of a relationship name, open Schema Builder and click the field. The Child Relationship Name appears on the field properties.
- Customise self-referencing relationships
Self-referencing lookups (ParentId on Account) need explicit naming. The platform default is rarely readable; rename to something clear like ChildAccounts.
- Default Child Relationship Names with awkward numbering confuse developers. Rename at field creation.
- SOQL subqueries error on non-existent relationship names. Inspect through Schema Builder if uncertain.
- Deep subqueries can return huge child lists and hit governor limits. Filter the subquery or use separate queries for large data sets.
- Polymorphic relationships (Task.WhatId) expose Child Relationship names per parent object. Apex code needs care to handle the parent type appropriately.