What is the _track proprerty of mail.thread model used for ?

In the mail.thread model _trackpropety is defined. It as the following doc `

# Automatic logging system if mail installed
# _track = {
#   'field': {
#       'module.subtype_xml': lambda self, cr, uid, obj, context=None: obj[state] == done,
#       'module.subtype_xml2': lambda self, cr, uid, obj, context=None: obj[state] != done,
#   },
#   'field2': {
#       ...
#   },
# }


But his puprose and usage is not really clear.

_track on the object is used to track events related to a document (an invoice has been paid, an opportunity is won, a task is blocked, ...). Users can follow events, represented by a mail.message.subtype, on any object.

It's different from the track_visibility attribute that you can define on a field which is used to track changes on this field. (e.g. Stage : Proposition -> Negociation)

Both _track and track_visibility produces messages on the document. Your object need to inherit from mail.thread.

If an object is inherited from 'mail.thread' then _track is used to send notifications. Therefore 'module.subtype_xml' is the related "Message Subtype". These subtypes have to be declared in XML. Here is an example:

<record id="subtype_xml" model="mail.message.subtype">
    <field name="name">Relevant Fields</field>
    <field name="res_model">project.issue</field>
    <field name="default" eval="True"/>
    <field name="description">The issue has been closed.</field>

Then whenever the field "field" is updated, all subtypes ("subtype_xml", "subtype_xml2") of this field are processed.

This means that the related method (in this example: lambda ...) is called and if the result is True, then for all users which follow this object and have checked the subtype a notification is created.

In the user preferences every user can choose whether he/she wants to be updated by email in case of new notifications.

You can also set a mail.message.subtype that depends on an other to act through a relation field. Here is an exemple from crm for Sales Teams crm.case.section using the section_id m2o in crm.lead:

    <record id="mt_lead_won" model="mail.message.subtype">
        <field name="name">Opportunity Won</field>
        <field name="res_model">crm.lead</field>
        <field name="default" eval="False"/>
        <field name="description">Opportunity won</field>

    <record id="mt_salesteam_lead_won" model="mail.message.subtype">
        <field name="name">Opportunity Won</field>
        <field name="res_model">crm.case.section</field>
        <field name="parent_id" eval="ref('mt_lead_won')"/>
        <field name="relation_field">section_id</field>

This allows a user to follow all "Opportunities Won" that are in a specific sales team. The user follow the event "Opportunity Won" on a sales team and he will become automatically follower of all leads/oppotunities of this sales team and _track event.

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