Relationships, Conceptually: Lookup vs Master-Detail
By the end of this lesson, you'll be able to:
- Explain what a relationship between two objects means
- Distinguish Lookup relationships from Master-Detail relationships at a conceptual level
- Predict what happens to child records when a parent record is deleted, under each relationship type
Prerequisites: "Standard Objects vs Custom Objects"
Why relationships exist
Real business data is never isolated. A Contact doesn't float on its own — it belongs to a company (Account). A Case doesn't exist in a vacuum — it was raised by someone (Contact) about something. Relationships are how Salesforce expresses "this record is connected to that one."
This is also exactly what makes SOQL relationship queries and Apex's handling of related records possible — you'll see this concept again constantly starting in Module 3's data modules and Module "SOQL."
Lookup: a loose connection
A Lookup relationship is like writing someone's employer's name on a business card — it's a reference, but the two things are otherwise independent. If the company goes out of business, your business card doesn't get automatically shredded.
In Salesforce terms: if you delete the parent record in a Lookup relationship, the child record survives (though the lookup field on it becomes blank, or in some configurations, deletion of the parent is blocked entirely if children still reference it).
Master-Detail: ownership
A Master-Detail relationship is tighter — the child genuinely belongs to, and depends on, the parent. Think of it like the relationship between a shipping order and its line items: a line item without an order makes no sense on its own.
In Salesforce terms: deleting the parent (master) record in a Master-Detail relationship deletes every related child record too — this cascading behavior is the single most important practical difference to remember.
A visual comparison
LOOKUP MASTER-DETAIL
------- --------------
Account (parent) Invoice__c (master)
| loosely referenced by | owns completely
v v
Contact (child) Invoice_Line__c (detail)
Delete Account? Delete Invoice__c?
Contact survives. Invoice_Line__c records
(lookup field blanks) are deleted too.A real business example: Manufacturing
A manufacturer might have a Master-Detail relationship from Production_Batch__c (master) to Quality_Check__c (detail) — a quality check genuinely doesn't mean anything without the batch it belongs to, so if a batch record is ever deleted, its quality checks should go with it. Meanwhile, Production_Batch__c might have a Lookup to Contact for "supervising engineer" — the batch should survive even if that engineer's contact record is later removed from the system.
Exercise
For a Customer Support scenario, would you make the relationship between Case and Case Comment a Lookup or a Master-Detail? Explain your reasoning using the 'does the child make sense without the parent' test from this lesson.
Show hint
Does a comment make sense floating around with no Case to belong to?
Relationships, Conceptually: Lookup vs Master-Detail Quiz
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Summary
A relationship connects one object's records to another's — a Contact belongs to an Account, an Opportunity belongs to an Account. Lookup relationships are loose connections; Master-Detail relationships are tight, ownership-style connections where the child's fate depends on the parent.