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Creative Querying for Many Records Using Collections

I have a query that i want to run which only returns records whose custom string field value is not within the keyset of a map. 

 

The trick is populating the map with thousands of records.

 

The problem i'm having is i'm running into heap size limits while trying to populate the map. If i can just populate that map within one instantiation/population line of code, i can avoid the heap size exception.

 

Example below:

 

Map<String, Custom_Object__c> myMap = new Map<String, Custom_Object__c>([Select Custom_Field__c From Custom_Object__c Where Custom_Field__c != null limit 50000]);

 

This will instantiate & populate the map with key value pairs on one line, and i avoid the heap exception. However the key for the map is the id of the records, and the value is the record. What i need as the key is the field 'Custom_Field__c'. 

 

Does anyone know how to accomplish the instantiation/population of a map on a single line of code where the key (or value for that matter) is a specific field value?

 

What i'm trying to accomlish is using that maps keys or values as a filter for another query for another object.

 

Thanks for the help in advance.

devendra dhakadevendra dhaka

Map<String, Custom_Object__c> myMap = new Map<String, Custom_Object__c>

for( Custom_Object__c co : [Select Custom_Field__c From Custom_Object__c Where Custom_Field__c != null ])

          myMap.put(co.id,co);

 

 

This might help. Even you won't have to put limit on records,