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I have a junction object(JuncObj) with a custom object(ParentObj1) and contact as the masters. I've created a separate obj(NewChildObj) that is supposed to be a child object of JuncObj. When I create a master detail relationship on NewChildObj, JuncObj doesn't appear on the list of object. What could be the problem? When I try this as a lookup, JuncObj is available. I really need NewChildObj as a child object with a master detail relationship so that I can make use of rollup summaries but I can't seem to figure why this is not allowed. I've read you cannot have more than 2 master detail relationships on an object, but does this mean a junction object cannot have a child object via a master detail relationship?

  • This is a limitation from salesforce. If an object is child in any of the master detail relationship, it can atmost participate in 2 master detail relationship. What you can do is, change the MD relationship to lookup for the JuncObj->ParentObj1 and JuncObj->Contact, if you don't need the MD relationship there. – Santanu Halder Jan 24 '17 at 9:50
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As per the help article:

Custom junction objects can't have detail objects. That is, a custom junction object can't become the master object in a multilevel master-detail relationship.

I strongly suggest you to go through the full article and understand all other considerations.

For your case, you can create junction object using lookup relationship instead of M-D, and then create M-D between NewChildObj and JunctionObj. Does that work?

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  1. Create a lookup from the child object to the parent object.

  2. Populate ALL the records with a valid lookup value to the parent.

  3. Change the Lookup relationship to a Master - Detail relationship. This is only allowed if ALL RECORDS HAVE A VALID LOOKUP. Otherwise you'll get the error you describe.

Also please check below link:

https://stackoverflow.com/questions/11088346/salesforce-can-a-junction-object-be-referenced-as-a-parent-of-other-object-wit

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