Django is a high-level Python Web framework that enables rapid development of secure and maintainable websites. Built by experienced developers, Django takes care of much of the hassle of Web development, so you can focus on writing your app without needing to reinvent the wheel.
To filter data in Django you utilize the ‘filter()’ method provided by the Django Object-Relational Mapping (ORM), Django’s database interface. The filter() method allows you to filter the records based on the provided arguments. Syntax: Model.objects.filter(conditions)
For example, if you have a model called “Person”, and you want to filter out all persons who are less than 18 years old, you would do:
```
underage_people = Person.objects.filter(age__lt=18)
```
This would return a QuerySet containing all people whose age is less than 18. Note the use of ‘**lt’ suffix which denotes ‘Less Than’. There are many such lookups you can use for comparisons, for example, ‘**gt’ for ‘Greater Than’, ‘**lte’ for ‘Less Than or Equal To’, ‘**gte’ for ‘Greater Than or Equal To’, etc.
You can also chain multiple conditions. For example:
```
old_tall_people = Person.objects.filter(age__gt=60, height__gt=180)
```
This would return all people who are older than 60 and taller than 180 cm.
If you want to filter using OR logic, you need to use Q objects:
```
from django.db.models import Q
old_or_tall_people = Person.objects.filter(Q(age__gt=60) | Q(height__gt=180))
```
This will return all people who are either older than 60 or taller than 180 cm.
For detailed documentation on querying data in Django, you can refer to [Django’s official documentation](https://docs.djangoproject.com/en/3.2/topics/db/queries/). It provides a comprehensive guide on how to retrieve data using Django ORM, including complex queries and relationships.
Please note that all of these operations might fail if the data types in the arguments do not match with the data types of the corresponding fields in the model, so always ensure that they are compatible.
In regards to performance, although Django’s ORM is very powerful, it’s recommended to retrieve only the necessary data to optimize your operations. The above examples are simple single table queries, but Django ORM is capable of performing complex joins, aggregations and even raw SQL queries when necessary.
Remember, every time you make a query, Django translates it to SQL, connects to the database, executes the SQL and returns the results. All these take resources so make sure to optimize your queries as much as possible.
The most important thing to remember is that these are ‘lazy’ queries, i.e., the actual SQL isn’t run until absolutely necessary. So you can continue to refine your queries until the very last moment.
