title: No visible @interface for 'HODatabasesManager' declares the selector 'db'
body: <p>The error is: No visible @interface for 'HODatabasesManager' declares the selector 'db'.</p>
  
  <pre><code>#import "BaseDB.h"
  
  @implementation BaseDB
  
  - (void)executeInBackgroundSuccess:(void (^)(id))success {
      dispatch_async(backgroundQueue(), ^{
          id result = [self executeQuery];
          dispatch_async(dispatch_get_main_queue(), ^{
              if (success) {
                  success(result);                
              }
          });
      });
  }
  
  - (FMDatabase *)fmDatabase {
      return [[HODatabasesManager shareInstance] db];
  }
  
  - (id)executeQuery {
      return nil;
  }
  
  dispatch_queue_t backgroundQueue() {
      static dispatch_once_t queueCreationGuard;
      static dispatch_queue_t queue;
      dispatch_once(&amp;queueCreationGuard, ^{
          queue = dispatch_queue_create("vn.conglv.dbqueue", 0);
      });
      return queue;
  }
  @end
  </code></pre>
  
  <p>How can I fix this?</p>