Fixes tiny documentation error. Adds possibility to add custom validation methods to fields, e. g.:

class Customer(Document):
        country = StringField(validation=lambda value: value in ['DE', 'AT', 'CH'])

Replaced some str() with unicode() for i18n reasons.

mongoengine/fields.py

@@ -39,6 +39,8 @@ class StringField(BaseField):
         if self.regex is not None and self.regex.match(value) is None:
             message = 'String value did not match validation regex'
             raise ValidationError(message)
+        
+        super(StringField, self).validate(value)
 
     def lookup_member(self, member_name):
         return None
@@ -93,6 +95,8 @@ class URLField(StringField):
             except Exception, e:
                 message = 'This URL appears to be a broken link: %s' % e
                 raise ValidationError(message)
+        
+        super(URLField, self).validate(value)
 
 class EmailField(StringField):
     """A field that validates input as an E-Mail-Address.
@@ -130,7 +134,8 @@ class IntField(BaseField):
 
         if self.max_value is not None and value > self.max_value:
             raise ValidationError('Integer value is too large')
-
+        
+        super(IntField, self).validate(value)
 
 class FloatField(BaseField):
     """An floating point number field.
@@ -153,6 +158,8 @@ class FloatField(BaseField):
 
         if self.max_value is not None and value > self.max_value:
             raise ValidationError('Float value is too large')
+        
+        super(FloatField, self).validate(value)
 
 
 class DecimalField(BaseField):
@@ -187,6 +194,8 @@ class DecimalField(BaseField):
 
         if self.max_value is not None and value > self.max_value:
             raise ValidationError('Decimal value is too large')
+        
+        super(DecimalField, self).validate(value)
 
 
 class BooleanField(BaseField):
@@ -200,6 +209,8 @@ class BooleanField(BaseField):
 
     def validate(self, value):
         assert isinstance(value, bool)
+        
+        super(BooleanField, self).validate(value)
 
 
 class DateTimeField(BaseField):
@@ -208,6 +219,8 @@ class DateTimeField(BaseField):
 
     def validate(self, value):
         assert isinstance(value, datetime.datetime)
+        
+        super(DateTimeField, self).validate(value)
 
 
 class EmbeddedDocumentField(BaseField):
@@ -239,6 +252,8 @@ class EmbeddedDocumentField(BaseField):
             raise ValidationError('Invalid embedded document instance '
                                   'provided to an EmbeddedDocumentField')
         self.document.validate(value)
+        
+        super(EmbeddedDocumentField, self).validate(value)
 
     def lookup_member(self, member_name):
         return self.document._fields.get(member_name)
@@ -315,6 +330,8 @@ class ListField(BaseField):
             [self.field.validate(item) for item in value]
         except Exception, err:
             raise ValidationError('Invalid ListField item (%s)' % str(err))
+        
+        super(ListField, self).validate(value)
 
     def prepare_query_value(self, op, value):
         if op in ('set', 'unset'):
@@ -359,6 +376,8 @@ class DictField(BaseField):
         if any(('.' in k or '$' in k) for k in value):
             raise ValidationError('Invalid dictionary key name - keys may not '
                                   'contain "." or "$" characters')
+        
+        super(DictField, self).validate(value)
 
     def lookup_member(self, member_name):
         return BaseField(db_field=member_name)
@@ -374,6 +393,8 @@ class GeoLocationField(DictField):
         
         if len(value) <> 2:
             raise ValidationError('GeoLocationField must have exactly two elements (x, y)')
+        
+        super(GeoLocationField, self).validate(value)
     
     def to_mongo(self, value):
         return {'x': value[0], 'y': value[1]}
@@ -443,6 +464,8 @@ class ReferenceField(BaseField):
 
     def validate(self, value):
         assert isinstance(value, (self.document_type, pymongo.dbref.DBRef))
+        
+        super(ReferenceField, self).validate(value)
 
     def lookup_member(self, member_name):
         return self.document_type._fields.get(member_name)
@@ -506,10 +529,12 @@ class BinaryField(BaseField):
         return pymongo.binary.Binary(value)
 
     def to_python(self, value):
-        return str(value)
+        return unicode(value)
 
     def validate(self, value):
         assert isinstance(value, str)
 
         if self.max_bytes is not None and len(value) > self.max_bytes:
             raise ValidationError('Binary value is too long')
+        
+        super(BinaryField, self).validate(value)