Our case definition contains a billion custom fields, and I appear to be fine SELECTing FieldName__c in an API call. But I want to get a field, Case_Assignee__c which is defined as a reference. Using it always says there is no such field despite it being in the describe for Case.
The field would be a user, just like Owner is, but Case_Assignee__c.Name fails along with any other permutation I can think of. So I guess I'm missing some key Salesforce magic knowledge. Any pointers?
When trying to query for data on other, related records (usually on a different SObject but you can have a lookup relationship between records of the same SObject), you need to change the __c of the relationship field to __r.
E.g.
SELECT Id, Name, CreatedDate, Special_Owner__r.Name FROM Opportunity That's true for any custom relationship field.
For standard relationship fields, the API name is usually of the form <object name>Id. E.g. OpportunityId on OpportunityLineItem, OwnerId on nearly all SObjects, etc...
For those, you'd just drop the "Id" part.
SELECT Id, Name, Opportunity.Name, Opportunity.StageName FROM OpportunityLineItem Do note that this is specific to child -> parent queries, i.e. traversing upwards in the relationship hierarchy. Querying downwards in the hierarchy (parent -> children) also needs you to use __r, but there's a bit more to it and that doesn't seem to be relevant to this particular question.
More detailed/thorough information can be found in the Relationship Queries section of the SOQL/SOSL Reference.
__cinstead of__rwhen you are trying to reference data on a related object. I.e. you should be usingMy_Relationship_Field__r.Related_Field__c.