diff --git a/relational/relation.py b/relational/relation.py
index 82ce1e9..b928113 100644
--- a/relational/relation.py
+++ b/relational/relation.py
@@ -100,18 +100,6 @@ class relation (object):
             ','.join(self.header.attributes) , ','.join(other.header.attributes)
             ))
 
-    def _autocast(self, string):
-        '''Depending on the regexp matched by the string,
-        it will perform automatic casting'''
-        if len(string) > 0 and string.isInt():
-            return int(string)
-        elif len(string) > 0 and string.isFloat():
-            return float(string)
-        elif len(string) > 0 and string.isDate():
-            return rdate(string)
-        else:
-            return string
-
     def selection(self, expr):
         '''Selection, expr must be a valid boolean expression, can contain field names,
         constant, math operations and boolean ones.'''
@@ -121,7 +109,7 @@ class relation (object):
         for i in self.content:
             # Fills the attributes dictionary with the values of the tuple
             for j in range(len(self.header.attributes)):
-                attributes[self.header.attributes[j]] = self._autocast(i[j])
+                attributes[self.header.attributes[j]] = i[j].autocast()
 
             try:
                 if eval(expr, attributes):
@@ -448,7 +436,7 @@ class relation (object):
         # new_content=[] #New content of the relation
         for i in self.content:
             for j in range(len(self.header.attributes)):
-                attributes[self.header.attributes[j]] = self._autocast(i[j])
+                attributes[self.header.attributes[j]] = i[j].autocast()
 
             if eval(expr, attributes):  # If expr is true, changing the tuple
                 affected += 1
diff --git a/relational/rtypes.py b/relational/rtypes.py
index 4698b6b..d0b9bed 100644
--- a/relational/rtypes.py
+++ b/relational/rtypes.py
@@ -28,6 +28,25 @@ import re
 class rstring (str):
 
     '''String subclass with some custom methods'''
+    def autocast(self):
+        '''
+        Returns the automatic cast for this
+        value.
+        '''
+        try:
+            return self._autocast
+        except:
+            pass
+
+        self._autocast = self
+        if len(self) > 0:
+            if self.isInt():
+                self._autocast = int(self)
+            elif self.isFloat():
+                self._autocast = float(self)
+            elif self.isDate():
+                self._autocast = rdate(self)
+        return self._autocast
 
     def isInt(self):
         '''Returns true if the string represents an int number
@@ -35,16 +54,10 @@ class rstring (str):
         the following regexp:
         r'^[\+\-]{0,1}[0-9]+$'
         '''
-        try:
-            return self._isint
-        except:
-            pass
-
         if re.match(r'^[\+\-]{0,1}[0-9]+$', self) == None:
-            self._isint = False
+            return False
         else:
-            self._isint = True
-        return self._isint
+            return True
 
     def isFloat(self):
         '''Returns true if the string represents a float number
@@ -52,16 +65,10 @@ class rstring (str):
         the following regexp:
             r'^[\+\-]{0,1}[0-9]+(\.([0-9])+)?$'
         '''
-        try:
-            return self._isfloat
-        except:
-            pass
-
         if re.match(r'^[\+\-]{0,1}[0-9]+(\.([0-9])+)?$', self) == None:
-            self._isfloat = False
+            return False
         else:
-            self._isfloat = True
-        return self._isfloat
+            return True
 
     def isDate(self):
         '''Returns true if the string represents a date,