Surveyforce - Link Survey to case

Question

I'm trying to set a trigger to link the results of a survey to the relevant case or chat transcript. It seems SurveyForce was not meant to do that but I tried adding a question on the survey that stores the Chatkey. And here is the code of the trigger :

trigger Test on LiveChatTranscript (before insert) 
{
    set<string> allSurveyStrings = new set<string>();

    for (SurveyTaker__c b : trigger.new)
       {
       allSurveyStrings.add(b.Response);
       }


    for (LiveChatTranscript a: Trigger.new)
       {

       Id SurveyTaker__c= [SELECT id FROM SurveyTaker__c WHERE Response.contains(a.chatKey) IN : allSurveyStrings]

       a.SurveyTaker__c = Id SurveyTaker__c;
       }

}

However the building fails everytime saying "Didn't understand relationship 'a' in field path"

Any help please ?