[SalesForce] Render Opportunity name from LookUp Relationship

I have a Custom Object that has a reference to Opportunities, I originally had it set up with a Master-Detail relationship but then after reading this and noting that parent objects can't be referenced, as well as the Master-Detail relationship only storing the ID, I switched the field type to a LookUp one.

So now my object's custom field's datatype is: Lookup(Opportunity) and I'm trying to make a VisualForce page that uses the StandardController (with reference to my custom object) render the name of the Opportunity. But the relationship doesn't seem to work and I just get unknown property String.name when I try. Can someone point me in the right direction?

Here's a bit of sample code:

<apex:page standardController="CustomObj__c" recordSetVar="objs">
    <apex:pageBlock title="">
         <apex:pageBlockTable value="{! objs}" var="item" >
            <apex:column headerValue="Opp. Name">
                <apex:outputText value="{! item.opp_to_exp__c.Name}"/>
            </apex:column>
            <apex:column headerValue="Opportunity">
                <apex:outputText value="{! item.opp_to_exp__c}"/>
            </apex:column>
        </apex:pageBlockTable>
    </apex:pageBlock>
</apex:page>

So opp_to_exp__c is API name for the field, and I've ripped out the specifics of the names and such besides that API name. I have no trouble rendering the other fields up of the object, just this lookUp one it seems.

The error message suggests to me that the opp_to_exp__c is still just a String for some reason even though I've changed the data type to be a LookUp one.

Can anyone help? I've spent over a few hours reading documentation and trying to find the right KeyTerms to search for on here with no avail.

Best Answer

Did you try replacing __c with __r i.e.

item.opp_to_exp__r.Name

That's typically how you reference related objects

Data Model and Terminology

Given an object Parent__c and object Child__c with a lookup to Parent__c.

Field Name

API Name for the lookup field on Child__c, i.e. Child__c.Parent_Lookup__c

Parent Relationship Name

Use to reference fields on the parent from the child object, i.e. String parentName = child.Parent_Lookup__r.Name. Derived from field name.

Child Relationship Name

Used to reference children from the parent, i.e. List<Child__c> children = parent.Children__r;

Uniqueness Constraints

Child relationship names must be unique for the parent. Meaning that there can't be more than one lookup to the parent object (doesn't matter what the child object is) for each child relationship name.

Conversely, parent relationship names, must be unique for each child object. Meaning there can't be more than one lookup on the child object with the same name (doesn't matter what the parent object is).

Examples

So if you had two account lookups on the case object, you couldn't use Cases__r for both child relationship names and instead would need to change one to Cases1__r. You'll notice that Salesforce automatically adds a numeral for you if the parent object already has a child relationship name with the child object's plural label.

Discussion

If it helps, you can think about whether something would need to be unique or not, by thinking about whether it would lead to a SOQL statement that could be interpreted two different ways which would make life for the database pretty tough.

Gotchas - Non-Unique Parent and Child Relationship

As @bryan points in his answer Salesforce doesn't check that a field relationship doesn't have the same base as a child relationship. This can cause issues it can't tell if obj.Value__r is looking for the custom field Value__c or the child relationships Value__r. By convention child relationships are plural so you'd need to change the default value to a singular version for this to happen.

[SalesForce] Why can’t I create a master-detail relationship between Product and the custom object

I can't offer you any insight into why you cannot do this, you just can't unfortunately, it's a seemingly arbitrary limitation of the platform and the Product2 object. Unless someone who knows about the internals of the platform comes across this I doubt you'll ever get a definitive answer.

In terms of working around it though, the only way to achieve this is to create your lookup as a standard non-required Lookup and then enforce that the value of this lookup cannot be blank using a validation rule.

Your validation rule will look something like this:

ISBLANK(ProductId__c)

If the Product is deleted then this will result in the value of the field being cleared, as per the default 'Clear the value of this field.' behaviour, which will prevent you updating the child record until a valid lookup ID is entered.

If you want to change this behaviour you'll have to write a before delete trigger on Product2 with your desired logic.