[SalesForce] Data Import Wizard doesnt show custom field of a custom object

It does not have to be defined on the child and the parent. The child will have the actual account relationship field (e.g., Account__c on your custom object, or whatever your child record is). The Account will have the External ID that identifies it. The child load file will then have the external ID in it instead of the Account ID. This is one reason for having an external ID, so that you can link these records without using the SF ID.

Assume that you have an external ID field called customId on the Account (or more generically the MasterObject). Here's what to do when you are importing the child:

1. Step 3 (I think?). Specify Relationships On the "Which MasterObject field are you including in your file to fulfill the Master/Detail requirement" select your customId field.

2. Step 5 (I think?). Field Mapping This is where it is not super obvious. The UI automatically maps the customID Import Field (i.e., CSV field) to the customID Salesforce.com field (i.e., Object field). That is NOT what you want. Instead change the Salesforce.com field to actually be the MasterObject field, eventhough the field in the CSV is not actually the ID of the MasterObject.

The key is the mapping of the customID field to the MasterObject field instead of accepting the default of it being mapped to the customID field.

You can do the same thing using the Data Loader. Instead of choosing Insert you must choose Upsert. See: https://developer.salesforce.com/docs/atlas.en-us.dataLoader.meta/dataLoader/?title=page/Additional_detail. The key here is doing an Upsert and then on the mapping page use relationship field, i.e., object__r:id_c .

(The above is similar to this post.)

Whenever you update your account make sure to click the box update existing account information and that should do the trick!