Alrond's technoblog

For the storage of the element state will be sometimes used the bitmasks. For example, as state - read/unread comment to article for each user. Although it would be better for this case, to save the time of the last view of the entry, here can arise also other tasks, in which must be remembered the binary data for elements, that don't include the timefield.

You can use the M2M-connection between a user and object. But the drawback is essential resources for index and the number of entries, if there are many users and objects. Here it would make sense to use the bit mask, without this shortcoming.

The idea is that there is only one entry in the databasa for each user, and the number of bytes in this entry and the number of bits in the byte determine the state of certain element. Normally Django has no model, that supports the blob-elements in the database (binary data). That is why I have used the usual textfield, its one symbol determines the state for four elements. This means, the entry of 250 bytes gives us the information about the state of 1000 elements.

For example, there are 10,000 jokes (this method is appropriate, if there are many elements, it means rather, the articles are small :)
Let there are 100,000 users. Then our connection model looks like this:

class Readed(models.Model):
  user = models.ForeignKey(User, related_name='readed')
  readed_flags = models.TextField(blank=True)

Number of entries is equal to the number of users, that is 100,000
A size of readed_flags is equal at most to 2500 bytes. (10.000/4)
With our solution we don't need to save all 2500 bytes, but only amount of bytes, that is the largest ID of the read article divided by 4.

So, we have 2 functions: is_readed(id) и set_readed(id).
is_readed(id) gives back the state (true/false) for the certain ID.
set_readed(id) set the state to "true".
After set_readed(), the changes shouldn't be forgotten to be saved in the database through save().
Within the function set_readed() I didn't put save(), so that it would be possible, to activate the flags for large amount of articles quickly and to save changes in the database all at once.

Here is the model code:

READED_FLAG = [1,2,4,8,0]
class Readed(models.Model):
  user = models.ForeignKey(User, related_name='readed')
  readed_flags = models.TextField(blank=True)

  def dec2hex(self,n):
    return "%X" % n
  
  def hex2dec(self,s):
    return int(s, 16)
  
  def is_readed(self,num):
    if num < 1:
      raise ValueError("Must be positiv and not null")
    byte_position, bit_position = divmod(num-1,4)
    notpresent = byte_position + 1 - len(self.readed_flags)
    if notpresent:
      return False
    four_status = self.hex2dec(self.readed_flags[byte_position])
    return (four_status & READED_FLAG[bit_position]) != READED_FLAG[4]
  
  def set_readed(self,num):
    if num < 1:
      raise ValueError("Must be positiv and not null")
    byte_position, bit_position = divmod(num-1,4)
    notpresent = byte_position + 1 - len(self.readed_flags)
    for i in xrange(notpresent):
      self.readed_flags += "0"
    if notpresent:
      four_status = 0
    else:
      four_status = self.hex2dec(self.readed_flags[byte_position])
    four_status_new = self.dec2hex(four_status|READED_FLAG[bit_position])
    readed_flags_new = self.readed_flags[:byte_position]+ \
                    four_status_new+self.readed_flags[byte_position+1:]
    self.readed_flags = readed_flags_new

The application of this method is much broader, than just substitution of the M2M-model. It depends on your imagination.

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