.. _table_data:

Populating a table with data
============================

Tables can be created from a range of input data structures. If you have seen the
tutorial you will have seen a ``QuerySet`` being used, however any iterable that
supports :func:`len` and contains items that exposes key-based access to column
values is fine.


List of dicts
-------------

In an example we will demonstrate using list of dicts. When defining a table
it is necessary to declare each column::

    import django_tables2 as tables

    data = [
        {"name": "Bradley"},
        {"name": "Stevie"},
    ]

    class NameTable(tables.Table):
        name = tables.Column()

    table = NameTable(data)


QuerySets
---------

If your build uses tables to display `~django.db.models.query.QuerySet` data,
rather than defining each column manually in the table, the `.Table.Meta.model`
option allows tables to be dynamically created based on a model::

    # models.py
    from django.contrib.auth import get_user_model
    from django.db import models

    class Person(models.Model):
        first_name = models.CharField(max_length=200)
        last_name = models.CharField(max_length=200)
        user = models.ForeignKey(get_user_model(), null=True, on_delete=models.CASCADE)
        birth_date = models.DateField()

    # tables.py
    import django_tables2 as tables

    class PersonTable(tables.Table):
        class Meta:
            model = Person

    # views.py
    def person_list(request):
        table = PersonTable(Person.objects.all())

        return render(request, "person_list.html", {
            "table": table
        })

This has a number of benefits:

- Less repetition
- Column headers are defined using the field's `~.models.Field.verbose_name`
- Specialized columns are used where possible (e.g. `.DateColumn` for a
  `~.models.DateField`)

When using this approach, the following options might be useful to customize
what fields to show or hide:

- `~.Table.Meta.sequence` -- reorder columns (if used alone, columns that are not specified are still rendered in the table after the specified columns)
- `~.Table.Meta.fields` -- specify model fields to *include*
- `~.Table.Meta.exclude` -- specify model fields to *exclude*

These options can be specified as tuples. In this example we will demonstrate how this can be done::
    
    # tables.py
    class PersonTable(tables.Table):
        class Meta:
            model = Person
            sequence = ("last_name", "first_name", "birth_date", )
            exclude = ("user", )

With these options specified, the columns would be show according to the order defined in the `~.Table.Meta.sequence`, while the ``user`` column will be hidden.

Performance
-----------

Django-tables tries to be efficient in displaying big datasets. It tries to
avoid converting the `~django.db.models.query.QuerySet` instances to lists by
using SQL to slice the data and should be able to handle datasets with 100k
records without a problem.

However, when performance is degrading, these tips might help:

1. For large datasets, try to use `.LazyPaginator`.
2. Try to strip the table of customizations and check if performance improves.
   If so, re-add them one by one, checking for performance after each step.
   This should help to narrow down the source of your performance problems.
