Avoiding SOQL inside loop returning null

Question

Below code is returning null for cmap. I am trying to debug contacts as id, contact name pair inside for loop but getting null.

Map<Id, case> caseMap = new Map<Id,case>([SELECT id, accountId, priority from case LIMIT 1]);
list<Id> accountIds = new list<Id>();
for(Id c : caseMap.keyset()) {
   if(caseMap.get(c).priority != 'low'){
      accountIds.add(caseMap.get(c).id);
      system.debug('h:'+cmap);
   }
}
Map<Id, contact> cmap = new map<Id, contact>([select id, accountid from contact where accountid =: accountIds]); 

Answer 1

Your second query is to retrieve records from the collected Ids I guess. If so you have to use IN clause in your query.

Map<Id, case> caseMap = new Map<Id,case>([SELECT id, accountId, priority from case LIMIT 10]);
list<Id> accountIds = new list<Id>();
for(Id c : caseMap.keyset()) {
   if(caseMap.get(c).priority != 'low'){
      accountIds.add(caseMap.get(c).accountId);  //collect accountIds
   }
}
Map<Id, contact> cmap = new map<Id, contact>([select id, accountid 
                                              from contact where accountid IN : accountIds]); 

Answer 2

By the way, if you didn't need that case map or list of account ids for anything else, and you were able to replace LIMIT in the case query with WHERE, you could get the contact map in one step:

Map<Id, contact> cmap = new map<Id, contact>([
    select id, accountid 
    from contact 
    where accountid IN (SELECT accountId FROM case WHERE id = :someid)
]);