Error: Compile Error: Invalid bind expression type

Question

I have to check where the Ids exists in the List of Strings

In the following code, Dependent_Question__c is a text field which stores the record id. I am getting this in DQList and checking these list in the below query using IN operator and I am getting this error:

Error: Compile Error: Invalid bind expression type of MM_Survey_Question__c for column of type String at line 175 column 175

Why am I getting this error and how can I fix it?

List<MM_Survey_Question__c> DQlist=new List<MM_Survey_Question__c>();
       DQlist=[select Dependent_Question__c from MM_Survey_Question__c where MM_Survey__c=:surveyRef and Dependent_Question__c!=null];
       System.debug('DQ List---'+DQlist); 
       if(dependentQuestion!=null)
       {
        dependentQuestionOptions = new List<SelectOption>();
        List<MM_Survey_Question__c> qlst = [Select Id, Name From MM_Survey_Question__c where MM_Survey__c=:surveyRef and Type__c='Single Select' and Dependent_Question__c IN:DQlist];

Answer 1

The problem is in the final portion of your last query's WHERE clause:

Dependent_Question__c IN:DQlist

Since the Dependent_Question__c field on the MM_Survey_Question__c object is a Text (String) field, DQlist would need to be a Set or List of type String, rather than type MM_Survey_Question__c as it is from the query return. In order to extract the Dependent_Question__c field values from each of the MM_Survey_Question__c records in DQlist, you will need to iterate through the list and collect a set of strings to use in the WHERE clause.

List<MM_Survey_Question__c> DQlist=new List<MM_Survey_Question__c>();
DQlist= [select Dependent_Question__c from MM_Survey_Question__c where MM_Survey__c=:surveyRef and Dependent_Question__c!=null];
Set<String> dependentQuestions = new Set<String>();
for (MM_Survey_Question__c surveyQuestion : DQlist) {
    dependentQuestions.add(surveyQuestion.Dependent_Question__c);
}
System.debug('DQ List---'+DQlist); 
if(dependentQuestion!=null)
{
    dependentQuestionOptions = new List<SelectOption>();
    List<MM_Survey_Question__c> qlst = [Select Id, Name From MM_Survey_Question__c where MM_Survey__c=:surveyRef and Type__c='Single Select' and Dependent_Question__c IN:dependentQuestions];

Stepping back a bit, though, why are you repeating the same query on your second and last lines, with only the added filter of Type__c='Single Select'? Why not include the Type__c filter in the original query?