3.2 Relationship Types & Implications

3.2 Relationship Types & Implications

Quick Answer: Choose lookup for optional or loosely coupled parents, master-detail when the child cannot exist without the parent and you need cascade delete, inherited security, and roll-up summaries, and a junction object for many-to-many. External and indirect lookups reach external objects; hierarchical is a special self-relationship on User.

Relationship design is one of the highest-yield data modeling topics on Platform Developer I. The same business “link” can be modeled several ways; the platform enforces very different behavior for each.

Lookup Relationships

A lookup creates an optional (or optionally required) foreign-key style link from a child to a parent.

Characteristics:

• Child can usually exist without a parent if the lookup is not marked required
• Parent and child can have independent owners and sharing
• Deleting the parent does not cascade-delete children by default (you can configure clear vs restrict-style behaviors depending on field settings and object type; know the exam framing: lookups are loosely coupled)
• No roll-up summary fields on pure lookup parents (use Flow, Apex, or declarative workarounds)
• Related lists appear on the parent; the child stores the parent Id in a field ending in __c (custom) with relationship navigation via __r

When to use: Related but independent records (for example, optional related Account on a custom Project, or linking Case to a custom Asset when the asset is not a strict parent).

Master-Detail Relationships

A master-detail relationship tightly couples child to parent.

Characteristics:

• Child requires a parent (detail cannot be orphaned)
• Cascade delete: deleting the master deletes detail records
• Inherited sharing: detail access is controlled by the master’s sharing (detail does not have independent OWD in the usual master-detail sense)
• Detail records typically do not have their own owner independent of the master (ownership follows the master model)
• Master can define roll-up summary fields (COUNT, SUM, MIN, MAX) over detail records
• A custom object can have up to two master-detail relationships (the foundation of many junction designs)

When to use: True parent-child lifecycles—Invoice Lines under Invoice, Expense Items under Expense Report—where lines should not survive the parent and should roll up totals.

Many-to-Many (Junction Objects)

Salesforce has no first-class “M:N field.” You model many-to-many with a junction object that has two relationships to the parent objects—classically two master-detail relationships (for example, Job_Application__c junction between Candidate__c and Position__c).

Implications:

• Each junction row represents one association pair (plus association attributes: status, applied date)
• With two master-details, junction sharing and cascade behavior follow master-detail rules from both parents (understand deletion impact)
• Related lists on each parent show related counterparts through the junction
• SOQL often queries the junction and navigates both __r parents, or uses subqueries from either parent

Sometimes architects use lookup-based junctions when independent ownership or optional parents are required—but then you lose native roll-ups and cascade semantics. Exam scenarios that stress “must be deleted with parent” and “roll up count of related X” point to master-detail junctions.

Hierarchical Relationships

A hierarchical relationship is a special lookup limited to the User object (self-relationship). It models manager hierarchies and similar user trees without using a generic self-lookup pattern available on all objects. For custom objects, self-relationships are ordinary lookups to the same object (for example, parent Account uses the standard Account hierarchy; custom Team__c might look up another Team__c).

External Lookup and Indirect Lookup

When Salesforce Connect exposes external objects (data lives outside Salesforce):

• External lookup: Child (Salesforce or external) relates to a parent external object using the external object’s external ID-style key
• Indirect lookup: External child object relates to a standard/custom Salesforce parent by matching an External ID field on the parent

These relationships power hybrid architectures. They do not behave like in-org master-detail for cascade delete and roll-ups; treat external data as loosely coupled and latency-sensitive. Exam items often test whether you recognize that external/indirect lookups are the correct link type when one side is an external object.

Record Access, UI, and Related Lists

Relationship type shapes the user experience:

• Related lists on parent layouts show child records; master-detail and lookup both support related lists, but security may hide children the user cannot see
• Master-detail children appear only when the user can access the master (inherited model)
• Lookup children may be visible based on their own sharing even if parent visibility differs (and vice versa), which can surprise admins
• Detail pages show lookup fields as clickable links to parents

Apex and SOQL Implications

• Child-to-parent: SELECT Name, Account__r.Name FROM Invoice__c
• Parent-to-child subquery: SELECT Name, (SELECT Amount__c FROM Invoice_Lines__r) FROM Invoice__c
• Relationship API names (plural child relationship names) are set when the relationship field is created—wrong names break subqueries
• Master-detail cascades mean Apex deletes on parents can remove large child sets—watch governor limits and recursive triggers
• Inserting detail records requires parent Id; order of insert in tests and loaders matters

Reparenting Rules

Reparenting means changing a child’s parent pointer after insert.

• Lookup: Generally allowed (unless the field is restricted); child can move between parents
• Master-detail: Reparenting is disabled by default on the relationship; admins can enable Allow reparenting on the master-detail field when the business must move details between masters

Exam scenarios that say “child must always stay with the original parent” align with master-detail without reparenting; “reassign line items to another order” needs lookup or master-detail with reparenting enabled.

Choosing Quickly Under Exam Pressure

1. Optional parent or independent security → lookup
2. Required parent, cascade delete, roll-ups, inherit sharing → master-detail
3. Many parents to many children → junction object
4. User manager chain → hierarchical (User)
5. Parent or child is external object → external / indirect lookup

Get the relationship wrong and every later choice—roll-ups, sharing, Apex, imports—gets harder. Get it right and the platform does much of the work for you.