How to send a variable from one view to other one in OpenERP7?

I added a button to a form view of a model (event.event). This button redirects to the tree view of other model (event.registration). I am trying to pass a variable (event_id) from the first view to the second one.

Then, in the tree view of the model event_registration, I added a new option to the menu More (at the top of the page), using act_window. The option which I added redirects to the form view of another model (mail.compose.message).

What I need is to pass the variable event_id from the view of event.event to the view of mail.compose.message. How can I achieve this?

MODEL event.event:

XML

<record id="action_view_event_registers"
    model="ir.actions.act_window">
    <field name="name">View registers</field>
    <field name="type">ir.actions.act_window</field>
    <field name="res_model">event.registration</field>
    <field name="view_type">form</field>
    <field name="view_mode">tree</field>
    <field name="context">{'event_id': context.get('event_id', False),}</field>
    <field name="view_id" ref="event.view_event_registration_tree" />
    <field name="target">current</field>
</record>

<record id="event_extended_form_view" model="ir.ui.view">
    <field name="name">event.extended.form.view</field>
    <field name="model">event.event</field>
    <field name="inherit_id" ref="event.view_event_form" />
    <field name="type">form</field>
    <field name="arch" type="xml">
        <field name="speaker_confirmed" position="after">
            <button name="%(event_extended.action_view_event_registers)d"
                string="View registers" type="action" icon="gtk-find"
                context="{'event_id': active_id}"/>
        </field>
    </field>
</record>

MODEL event.registration:

XML

<act_window name="Invite partners to an event"
    res_model="mail.compose.message"
    src_model="event.registration"
    view_mode="form"
    multi="True"
    target="new"
    context="{'event_id': context.get('event_id'),}"
    id="invite_partner_to_event"/>

MODEL mail.compose.message:

PY

def _default_template(self, cr, uid, context=None):
    event_id = context.get('event_id', None)
    _logger.info(event_id)
    return False

_defaults = {
    'template_id': _default_template,
}

As you can see, I'm doing nothing, only trying to print the event_id in the log file. However, it is returning None.

Can anybody help me here, please? I'm not sure if this is the right way to achieve my target.