from google.appengine.ext import db

import md5
import base64

class UniqueConstraint(db.Model):
    @classmethod
    def check(cls, model, **values):
        # Build a list of key names to test.
        key_names = []      
        concat = ""  
        for key in values:
            pair_str = '%s:%s' % (key, values[key])
            key_names.append(pair_str)
            concat += pair_str
        
        # "sharded" version now added - it creates a hash for each one of the
        # possible key combinations
        concat_hash = base64.b64encode(md5.new(concat).digest())
        
        # Create a pseudo-key for use as an entity group.
        parent = db.Key.from_path(model.kind(), 'unique-values' + concat_hash[0:2])


        def txn():
            result = cls.get_by_key_name(key_names, parent)
            for test in result:
                if test: return False
            for key_name in key_names:
                uc = cls(key_name=key_name, parent=parent)
                uc.put()
            return True

        return db.run_in_transaction(txn)