How to access and not hardcode the field ID for a custom object? Using Apex, to construct URLs

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Lets say you wanna create a new instance of a custom object, lets call this Custom Object Relationship_Detail__c.

You are trying to create a link where it takes you to the proper RecordTypeId and it prepopulates the first field of Relationship_Detail__c, this field is called First_Contact__c or First_Account__c, they are take in the Name of a account of contact.

In the new relationship page you can use firebug or inspect the source to find that the ID for this custom field is CF00No00000020GfM for First_Account__c and CF00No00000020GfN for First_Contact__c.
Cool, but this value changes between orgs, so i need to have access to this ID without hardcoding it.

How can i access this value?
How can I access the ID for a filed of a custom object?
Preferably not through using querys.

This is my code as right now:

public PageReference newRelationshipButtonClicked(){

String prefixForNewRelationship = Relationship_Detail__c.SObjectType.getDescribe().getKeyPrefix();

 Id recordTypeId;
 String  encodedObjectName ='';
 String typeOfObjectForFirst = '';


 if(firstObjectIsAccount){

  encodedObjectName = EncodingUtil.urlEncode(myAccount.Name, 'UTF-8');
  typeOfObjectForFirst = 'CF00No00000020GfM';

  if(secondObjectIsAccount){ //both are accounts

   recordTypeId = Schema.SObjectType.Relationship_Detail__c.getRecordTypeInfosByName().get('Account-Account Relationship').getRecordTypeId();


  } else{ //im account, person i wanna have a relation to is a contact

   recordTypeId = Schema.SObjectType.Relationship_Detail__c.getRecordTypeInfosByName().get('Account-Contact Relationship').getRecordTypeId();

  }

} else { //im a contact

  encodedObjectName = EncodingUtil.urlEncode(myContact.Name, 'UTF-8');      
  typeOfObjectForFirst = 'CF00No00000020GfN';

  if(secondObjectIsAccount){ //im a contact, other is an account

   recordTypeId = Schema.SObjectType.Relationship_Detail__c.getRecordTypeInfosByName().get('Contact-Account Relationship').getRecordTypeId();

  } else{ //we are both contacts

   recordTypeId = Schema.SObjectType.Relationship_Detail__c.getRecordTypeInfosByName().get('Contact-Contact Relationship').getRecordTypeId();

}

Answer

There are these two techniques for getting the field IDs programmatically:

This is assuming that the fields you are trying to default are custom object fields (which they appear to be from your URL example).

(As you probably already know there are several ways to get the record type ID including querying the RecordType object e.g. by DeveloperName.)

Attribution
Source : Link , Question Author : user10009 , Answer Author : Keith C

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