Introduction to data views¶
What is a data view?¶
- data view¶
A Python class that describes how to retrieve and display a set of data rows.
Each data view provides a set of display modes, a list of actions and layouts.
- table view¶
Old word for a tabular data view.
- tabular data view¶
A data view in a display mode that uses columns and rows to display the data (which is either grid mode or plain mode).
- model-based data view¶
A data view that is connected to database model.
In a Lino application you describe data views using Python classes. These Python classes are an abstract description of how to lay out your data. Depending on the front end this leads to similar but not identical results.
A same data view is used to render data interactively as a grid panel or on a printable document as a table.
Don't mix up models and data views: while your models describe how data is to be stored in the database, your data views describe how data is to be presented to end users.
Lino's "data views" are more general than Django's "views". With Lino you don't need to write Django views because Lino writes them for you. Every data view usually causes multiple Django views.
In Lino we differentiate between model-based data views and virtual tables. The former get their data directly from the database using a Django model. The latter have no database model, they get their data programmatically.
Implementation note: model-based table views are subclasses of
lino.core.dbtables.Table
(generally imported via its shortcut
dd.Table
), virtual tables are subclasses of
lino.core.tables.VirtualTable
(generally imported via its shortcut
dd.VirtualTable
). The classes have a common abstract base
class lino.core.tables.AbstractTable
.
The remainder of this tutorial concentrates on model-based table views, virtual tables have a tutorial on their own.
Illustration¶
To illustrate this, we will have a look at the
lino_book.projects.tables
demo application.
Here are the database models:
from lino.api import dd
from django.db import models
from django.core.exceptions import ValidationError
class Author(dd.Model):
first_name = models.CharField("First name", max_length=50)
last_name = models.CharField("Last name", max_length=50)
country = models.CharField("Country", max_length=50, blank=True)
def __str__(self):
return "%s, %s" % (self.last_name, self.first_name)
class Book(dd.Model):
author = dd.ForeignKey(Author, blank=True, null=True)
title = models.CharField("Title", max_length=200)
published = models.IntegerField(
"Published",
help_text="The year of publication")
price = models.DecimalField("Price", decimal_places=2, max_digits=10)
def full_clean(self):
super(Book, self).full_clean()
if self.published > 2000 and self.price < 5:
price = dd.format_currency(self.price)
msg = "A book from {} for only {}!".format(
self.published, price)
raise ValidationError(msg)
from .ui import *
And here are the data views:
from lino.api import dd
class Authors(dd.Table):
model = 'Author'
column_names = 'first_name last_name country'
detail_layout = """
first_name last_name country
BooksByAuthor
"""
class Books(dd.Table):
model = 'Book'
column_names = 'author title published *'
hide_sums = True
class RecentBooks(Books):
column_names = 'published title author'
order_by = ['published']
class BooksByAuthor(Books):
master_key = 'author'
column_names = 'published title'
order_by = ['published']
Data views must exist in your models.py
, in the same namespace as your
database models. But you might prefer to actually
define them in a separate file and import them into your models.py
by
saying:
from .ui import *
We recommend to name such a file ui.py
.
Note that using a desktop.py
file is deprecated.
Table views are subclasses of dd.Table
. You
don't need to instantiate them, Lino discovers them automatically at startup and
they are globally available at runtime in the lino.api.rt
module.
>>> from lino import startup
>>> startup('lino_book.projects.tables.settings')
>>> from lino.api import rt, dd
>>> rt.models.tables.Books
lino_book.projects.tables.ui.Books
>>> issubclass(rt.models.tables.Books, dd.Table)
True
There can be more than one data view for a given database model, but each data
view has exactly one model as its data source. That model is specified in the
model
attribute. For every database
model there should be at least one data view, otherwise Lino will generate a
default data view for it.
Much information about your data view is automatically extracted from the model: the columns correspond to the fields of your database model. The header of every column is the verbose_name of its field. The values in a column are of same data type for each row. So Lino knows all these things from your models.
The rows of a table can be sorted and filtered. These things are
done in Django on a QuerySet. Lino forwards them to their corresponding Django
methods: order_by
,
filter
and exclude
.
But here is something you cannot express on a Django model: which
columns are to be shown, and how they are ordered. This is defined by
the column_names
attribute, a simple string with a space-separated list of field names.
Data views can hold information that goes beyond a database model or a queryset.
For example we set hide_sums
to True on the Books
table because otherwise Lino would display a sum for
the "published" column.
Designing your data¶
Database models are usually named in singular form, data views in plural form.
Date views may inherit from other data views (e.g. BooksByAuthor
inherits
from Books
: it is basically a list of books, with the difference that it
shows only the books of a given author.
As a rule of thumb you can say that you need one data view for every data
window in your application. Each data view is a subclass of dd.Table
.
To define data views, you simply need to declare their classes. Lino discovers and analyzes them when it initializes. Data views never get instantiated.
Each data view must have at least one class attribute model
, which points to the model on which this view
will "work". Every row of a table represents an instance of its model. (This is
true only for database tables. Lino also has virtual tables, we will talk
about them in a later tutorial.
Since data views are normal Python classes, they can use inheritance. In our code BooksByAuthor inherits from Books. That's why we don't need to explicitly specify a model attribute for BooksByAuthor.
BooksByAuthor is an example of a slave table. It shows the books of a given Author. This given Author is called the "master" of these Books. We also say that a slave table depends on its master.
Lino manages this dependency almost automatically. The application developer
just needs to specify a class attribute master_key
. This attribute, when set, must be
a string containing the name of a ForeignKey field of the data view's
model
.
A data view can define attributes like filter
and order_by
, which you know from Django's
QuerySet API.
The columns of a data view¶
An important attribute of a data view is column_names
, which describes the columns to
show in
- class lino.core.tables.AbstractTable
- column_names = '*'
A string that describes the list of columns of this table.
Default value is
'*'
, which means to show all columns.Lino will automatically create a
lino.core.layouts.ColumnsLayout
from this. This string must not contain any newline characters because a ColumnsLayout's main panel descriptor must be horizontal.See also
setup_column()
andget_column_names()
.
- hidden_columns = frozenset()
If given, this is specifies the data elements that should be hidden by default when rendering this table. Example:
hidden_columns = "long_name expected_date"
Value : Application code should specify this as a single string containing a space-separated list of field names. Lino will automatically resolve this during server startup using
lino.core.utils.fields_list()
. The runtime value of this attribute is a set of strings, each one the name of a data element. Defaults to an empty set.Inheritance : Note that this can be specified either on a
Model
or on aTable
. Lino will make a union of both.
Wildcard columns¶
The asterisk ('*'
) in a column specifier is a wildcard and means "insert at
this point all columns that have not been named explicitly". It can be combined
with explicitly specified names. These wildcard columns
If '*'
is not present in the string, only explicitly named columns will be
available.
For example:
column_names = "name owner * date"
specifies that name and owner come first, followed by inserted columns and finally by date.
Virtual fields are not included as wildcard field unless they have
lino.core.fields.VirtualField.wildcard_field
set to True. This rule is
for performance reasons. Some virtual fields a rather heavy (e.g. the
navigation_panel
must query the
whole database to get all primary keys), and even when they are hidden, Lino has
to include wildcard fields in the result because the
end user might have enabled them.
Other table view attributes¶
But the table is even more than the description of a grid widget. It
also has attributes like detail_layout
, which tells it how to display
the detail of a single record in a form view.
Using tables without a web server¶
An important thing with tables is that they are independent of any front end. You define them once, and you can use them on the console, in a script, in a testcase, in a web interface or in a GUI window.
At this point of our tutorial, we won't yet fire up a web browser (because we want to explain a few more concepts like menus and layouts before we can do that), but we can already play with our data using Django's console shell:
$ python manage.py shell
The first thing you do in a shell
session is to import
everything from lino.api.shell
:
>>> from lino.api.shell import *
This imports especially a name rt
which points to the
lino.api.rt
module. rt
stands for "run time" and it
exposes Lino's runtime API. In our first session we are going to use
the show
method and the actors
object.
>>> rt.show(tables.Authors)
...
============ =========== =========
First name Last name Country
------------ ----------- ---------
Douglas Adams UK
Albert Camus FR
Hannes Huttner DE
============ =========== =========
So here is, our Authors
table, in a testable console format!
And here is the Books
table:
>>> rt.show(tables.Books)
...
================= ====================================== ===========
author Title Published
----------------- -------------------------------------- -----------
Adams, Douglas Last chance to see... 1990
Adams, Douglas The Hitchhiker's Guide to the Galaxy 1978
Huttner, Hannes Das Blaue vom Himmel 1975
Camus, Albert L'etranger 1957
================= ====================================== ===========
These were so-called master tables. We can also show the content of slave tables :
>>> adams = tables.Author.objects.get(last_name="Adams")
>>> rt.show(tables.BooksByAuthor, adams)
...
=========== ======================================
Published Title
----------- --------------------------------------
1978 The Hitchhiker's Guide to the Galaxy
1990 Last chance to see...
=========== ======================================
Before going on, please note that the preceding code snippets are
tested as part of Lino's test suite. This means that as a core
developer you can run a command (inv test
in case you are
curious) which will parse the source file of this page, execute every
line that starts with >>>
and verifies that the output is the same
as in this document. If a single dot changes, the test "fails" and
the developer will find out the reason.
Writing test cases is an important part of software development. It might look less funny than developing cool widgets, but actually these are part of analyzing and describing how your users want their data to be structured. Which is the more important part of software development.
Defining a web interface¶
The last piece of the user interface is the menu definition, located
in the __init__.py
file of this tutorial:
from lino.api import ad, _
class Plugin(ad.Plugin):
verbose_name = _("Tables")
def setup_main_menu(self, site, profile, m):
m = m.add_menu(self.app_label, self.verbose_name)
m.add_action('tables.Authors')
m.add_action('tables.Books')
Every plugin of a Lino application can define its own subclass of
lino.core.plugin.Plugin
, and Lino instantiates these objects
automatically a startup, even before importing your database models.
Note that a plugin corresponds to what Django calls an application. More about this in Introduction to plugins.
Exercises¶
Explore the application and try to extend it: change things in the code and see what happens.
You can interactively play around with the little application used in this tutorial:
$ go tables
$ python manage.py runserver
Some screenshots:


The fixtures/demo.py
file contains the data we used to fill our
database:
from lino.api.shell import *
from lino.utils.instantiator import Instantiator
def objects():
author = Instantiator(
'tables.Author', 'first_name last_name country').build
adams = author("Douglas", "Adams", "UK")
yield adams
camus = author("Albert", "Camus", "FR")
yield camus
huttner = author("Hannes", "Huttner", "DE")
yield huttner
book = Instantiator('tables.Book', 'title author published price').build
yield book("Last chance to see...", adams, 1990, '9.90')
yield book("The Hitchhiker's Guide to the Galaxy", adams, 1978, '19.90')
yield book("Das Blaue vom Himmel", huttner, 1975, '14.90')
yield book("L'etranger", camus, 1957, '6.90')
# yield book("Book", camus, 2001, '4.90')
Glossary¶
- wildcard column¶
A data element that has been inserted by a
*
and which is hidden by default. See Wildcard columns.- wildcard field¶
A database field that is candidate to becoming a wildcard column.