'Return  date of membership'

@api.depends('member_lines.account_invoice_line.invoice_id.state', 'member_lines.account_invoice_line.invoice_id.invoice_line_ids', 'member_lines.account_invoice_line.invoice_id.payment_ids', 'free_member', 'member_lines.date_to', 'member_lines.date_from', 'membership_state', 'associate_member.membership_state')
def _compute_membership_start(self):

    for partner in self:
        partner.membership_start = self.env['membership.membership_line'].search([('partner', '=', (partner.associate_member.id or partner.id)), ('date_cancel', '=', False)], limit=1, order='date_from').date_from
