<record id="custom_crm_activity_tree_view" model="ir.ui.view">
        <field name="name">activities</field>
        <field name="model">crm.activity</field>
    <field name="type">tree</field>
    <field name="arch" type="xml">
      <tree string="Activities list">
        <field name="partner_id"/>
        <field name="opportunity_id"/>
        <field name="activity_name"/>
      </tree>
        </field>
    </record>

   <record id="custom_crm_activity_form_view" model="ir.ui.view">
        <field name="name">activity form</field>
        <field name="model">crm.activity</field>
    <field name="type">form</field>
        <field name="arch" type="xml">
        <form string="Activity form" version="7.0" create="true" edit="true">
          <sheet>
            <group>
            <field name="partner_id" string="Customer"/>
            <field name= "opportunity_id" string="Opportunity"/>
            <field name="activity_name" string="Activity"/>
            </group>
          </sheet>
        </form>
        </field>
    </record>

class CrmActivity(models.Model):

    _name = 'crm.activity'
    _description = 'CRM Activity'
    _inherits = {'mail.message.subtype': 'subtype_id'}
    _rec_name = 'name'
    _order = "sequence"


    partner_id = fields.Many2one('res.partner', string='Customer', track_visibility='onchange', index=True,
        help="Linked partner (optional). Usually created when converting the lead.")
    opportunity_id = fields.Many2one('crm.lead', string='Opportunities', domain=[('type', '=', 'opportunity')])
    activity_name = fields.Char('Activity',default='activity name')