Add a has-instance-with-id function to check the existance of an instance. Also fix the count-instance function.

src/notepad/stored-objects.rkt

@@ -23,6 +23,7 @@
   save-instance
   save-instances
   find-instance-by-id
+  has-instance-with-id
   list-instances
   find-instances
   count-instances
@@ -270,6 +271,18 @@
     (set-struct:stored-instance-identifier! instance id)
     instance))
 
+; Check if an instance with the given id exists
+(define/contract (has-instance-with-id cls id)
+  (-> struct:stored-class? integer? boolean?)
+  (define repo (struct:stored-class-repository cls))
+  (define conn (struct:repository-connection repo))
+  (define row (query-row
+                conn
+                (format "select count(*) from ~a where rowid = ?"
+                  (struct:stored-class-name cls))
+                id))
+  (> (vector-ref row 0) 0))
+
 ; Return a list of instances
 (define (list-instances cls #:limit (limit #f) #:offset (offset #f) #:descending (desc #f))
   (let* ((repo (struct:stored-class-repository cls))
@@ -353,7 +366,9 @@
   (let* ((repo (struct:stored-class-repository cls))
          (conn (struct:repository-connection repo))
         )
-    (query-value conn (format "select count(*) from ~a" (struct:stored-class-name cls)))))
+    (vector-ref
+      (query-value conn (format "select count(*) from ~a" (struct:stored-class-name cls)))
+      0)))
 
 ; Store an instance
 ; Create a new instance in the repository