What is Master-detail relationship?

A master-detail relationship in Salesforce is a tightly coupled parent-child relationship where the detail (child) record cannot exist without its master (parent) record. The master controls the detail’s ownership, sharing, and lifecycle: deleting the master cascades to all its details, the detail inherits the master’s owner, and the OWD on the detail is locked to “Controlled by Parent.” Master-detail is the only relationship type that enables built-in roll-up summary fields.

What the relationship enforces

When you create a master-detail field on the child object pointing to the parent:

• The master-detail field is always required — you cannot save a child record without a parent.
• The child’s OwnerId field disappears — there is no separate owner; the parent’s owner is effectively the child’s owner.
• The child’s OWD is locked to “Controlled by Parent” — sharing settings on the child come from the parent.
• Deleting the master record cascades the deletion to all detail records (and their details, recursively up to 3 levels deep).
• The child appears in the master’s related list by default.
• The master object can have roll-up summary fields counting or summarizing child fields.

When master-detail is the right choice

Master-detail is appropriate when the child is conceptually part of the parent — when no business meaning exists for the child without the parent.

Classic master-detail relationships:

• Opportunity Line Item → Opportunity (standard, ships this way)
• Case Comment → Case (standard)
• Order Product → Order (standard)
• Invoice Line Item → Invoice (custom, common pattern)
• Project Task → Project (custom)
• Survey Question → Survey (custom)
• Attendee → Event Session (custom)

If you’re tempted to model an “Account → Opportunity” master-detail because you want a roll-up summary of Opportunity Amount on Account — don’t. Salesforce intentionally made that a lookup because Opportunities have independent ownership, lifecycle, and sharing from their Accounts. Modeling it as master-detail would be a serious data-model mistake.

Key constraints

Master-detail and roll-up summaries

Master-detail unlocks roll-up summary fields on the master. You can create up to 25 roll-up summary fields per object (limit may shift between releases — check current docs). They support:

• COUNT — number of detail records
• SUM — total of a numeric field on details
• MIN / MAX — smallest/largest value across details

You can filter the rolled-up records by criteria (e.g., only roll up Closed Won opportunities, only roll up open tasks).

Invoice (master)
  ├── Invoice Line Item (detail)
  │      Quantity__c, Unit_Price__c, Line_Total__c
  └── Roll-Up Summary on Invoice:
         Total_Amount__c = SUM(Invoice Line Item.Line_Total__c)
         Line_Count__c   = COUNT(Invoice Line Item)

Roll-up summaries are not available on lookup relationships — that’s the single biggest reason to choose master-detail when the data shape supports it.

Converting between lookup and master-detail

You can convert in either direction, but with strict conditions:

• Lookup → Master-Detail: every existing child record must have a value in the lookup field (no nulls). Salesforce blocks the conversion otherwise.
• Master-Detail → Lookup: any roll-up summary fields on the master that depend on the detail must be deleted first.

If you’re building a new object and might need roll-up later, starting with lookup and converting to master-detail is a common pattern — just be aware of the no-nulls constraint when the time comes.

Real-world scenario

“You need to model Quotes and Quote Line Items, and the Quote should show the total line amount automatically.”

Implementation:

1. Create custom object Quote__c (or use the standard Quote if appropriate).
2. Create custom object Quote_Line_Item__c with a master-detail field pointing to Quote__c.
3. On Quote__c, create a roll-up summary field: Total_Amount__c = SUM(Quote_Line_Item__c.Line_Total__c).
4. Optional: another roll-up Line_Count__c = COUNT(Quote_Line_Item__c).

When a user adds, deletes, or changes a line item, the totals on Quote update automatically — no flow, no trigger, no Apex required.

Verified against: Salesforce Help — Master-Detail Relationships and Roll-Up Summary Fields. Last reviewed 2026-05-17.