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.. _topcis-fields_on_change:
===================
on_change of fields
===================
Tryton allows developers to define methods that can be called once a field's
value has changed by the user this is the :ref:`ref-models-fields-on_change`
method. The method has the following name::
Model.on_change_<field name>
This is an instance method, an instance of ``Model`` will be created by
using the values from the form's fields specified by the ``on_change`` list
defined on the field.
There is also a way to define a method that must update a field whenever any
field from a predefined list is modified. This list is defined by the
:ref:`ref-models-fields-on_change_with` attribute of the field. The method
that will be called has the following name::
Model.on_change_with_<field_name>
Just like for the classic ``on_change``, an instance of ``Model`` will be
created by using the values entered in the form's fields specified by the
``on_change_with`` attribute.
on_change & on_change_with return values
----------------------------------------
The return value of the method called will depend of the type of the call that
occured. In case of an ``on_change`` the returned value will be a dictionary
whose keys are field names to be modified and whose values will be the
corresponding new value. In case of an ``on_change_with`` the returned value
will be the new value of the field.
Pay attention that the new value of a field differs according to its type.
Simple fields require the value to be of the same type as the field.
Relation fields require some more explanations:
- a ``field.One2Many`` or a ``field.Many2Many`` will accept either:
- a list of ``id`` denoting the new value that will replace all
previously ids.
- a dictionary composed of three keys: ``update``, ``add`` and
``remove``.
The ``update`` key has as value a list of dictionary that denotes the
new value of the target's fields. The lines affected by the change
are found using the ``id`` key of the dictionary.
The ``add`` key have as value a list of tuple, the first element is
the index where a new line should be added, the second element is a
dictionary using the same convention as the dictionaries for the
``update`` key.
The ``remove`` key will have as value a list of ids that will be
remove from the field.
Records that are not yet saved receive temporary ids (negative
integers) that can be used in with ``update`` or ``remove`` to
interact with them.
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