# Create a Tag field using Django-Select2.

The excellent framework – Select2, have had support for tags for a long time, but Django-Select2 lacked that, until now (version 4.2.0).

Tag fields are very much like any multiple value input field except that it allows users to enter values which does not exist in the backing model. So, that the users can add new tags.

For this purpose few new fields have been added to Django-Select2. They all have the suffix – TagField. Few widgets too have been added to auto configure Select2 Js to run in “tagging mode”.

You can see the full reference in docs – http://django-select2.readthedocs.org/en/latest/ref_fields.html and http://django-select2.readthedocs.org/en/latest/ref_widgets.html.

## Simple tag field implementation example

You can find this code in testapp too.

models.py:-

#!python
class Tag(models.Model):
tag = models.CharField(max_length=10, unique=True)

def __unicode__(self):
return unicode(self.tag)

class Question(models.Model):
question = models.CharField(max_length=200)
description = models.CharField(max_length=800)
tags = models.ManyToManyField(Tag)

def __unicode__(self):
return unicode(self.question)

forms.py:-

#!python
class TagField(AutoModelSelect2TagField):
queryset = Tag.objects
search_fields = ['tag__icontains', ]
def get_model_field_values(self, value):
return {'tag': value}

class QuestionForm(forms.ModelForm):
question = forms.CharField()
description = forms.CharField(widget=forms.Textarea)
tags = TagField()

class Meta:
model = Question

Above I am trying to create a form which the website users can use to submit questions. Things to note in TagField is that it is almost like we use any other AutoModel fields in Django-Select2, except that here we override one new method get_model_field_values().

When users submit a tag field, the usual validation runs, but with a twist. While checking if the provided value exists, if it is found not to exist then the field, instead of raising error, creates that value. However, the library does not know how to create a new value instance, hence it invokes create_new_value() to do that job.

Model version of the tag field already knows that it is dealing with Django models so it knows how to instantiate that, but does not know what values to set for the attributes. So it implements create_new_value() which in-turn invokes get_model_field_values() to get required attribute names mapped to their values.

## Before you continue

One key thing to remember is that unlike other widgets, you ideally should not be using tag widgets with normal Django fields. Django fields do not allow creation of a new choice value. However, if you want tagging support but do not want to allow users to create new tags then you can very much use the tag widget here and pair it up with normal Django fields. However, that may not be a good idea, since then UI would still allow creation of new tags, but on submit the user would get an error.

## Making it better

We can make this better and interesting. A typical tagging system should have the following features.

• Ability to detect misspellings. Peter Norvig’s essay on this is excellent. More information can be found on Stack Overflow.
• Use statistics to order the results. This very much useful when the tag counts ballon up. The statistics could be based on how many tags are frequently used together. Of course when you start a site you would not have any data, in that case for a period of time you can set the algo to only learn.
• Cache frequently used tags. This is a normal optimization technique which is frequently used. A memcache like layer is usually used to cache the DB data, and if the data is not found there, then a DB hit is made.

# Find one’s complement of a decimal number.

One’s complement of a binary number (1001_{2}) is (0110_{2}). That is, we simply flip the bits. This is easy to do in binary. However, if the number is in base 10, then do we do that. Convert that to base 2 then flip the bits and change it back to base 2? Maybe, but if the number is stored in native data types (like int) then we can simply use bitwise not to invert the bits, without any conversion.

In my case it wasn’t so simple. I had a really big number, which had to be stored in BigDecimal. Converting to base 2, back and forth too is very costly. So the efficient algo for that is…

(x’) is the one’s complement of (x) and it has (b) bits, or we are interested in only (b) bits of it.

\begin{align} &x + x’ = 2^{b+1} – 1 \tag{all ones for all b bits} \ &\Rightarrow x’ = 2^{b+1} – 1 – x \ \end{align}

So, the equivalent Java code for the above is…

public static BigDecimal onesComplement(BigDecimal n, int totalBits) {
return new BigDecimal(2).pow(totalBits + 1).subtract(BigDecimal.ONE).subtract(n);
}