If you look at activity and what it's connected to via the whatId, it's represented as a lookup. It would seem that polymorphic is closer to MDR.

Any idea what's going on there?

  • Seems like a valid point Apr 23, 2013 at 14:20
  • 3
    Polymorphic relationships are lookups, save for it can be a variety of sObjects, rather than fixed to one? Apr 23, 2013 at 15:43

1 Answer 1


Lookup is just a salesforce specific term for a foreign key relationship. Master-detail has a number of additional implications that generally are not true of polymorphic lookups.

For example, an object on the detail side of a MDR has no owner field, and is not able to participate in the sharing model; instead access to it is controller by the parent record. They're also able to use rollup summary fields and cross-object workflow.

While many polymorphic lookups cascade delete like MDRs do they don't always meet the full criteria for a MDR, and listing them as a lookup is more appropriate despite their special behavior.

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