rate_usd = self.env['res.currency'].search([('name', '=', 'USD')], limit=1).rate

I got the rate on a particular date as:

currency_id.with_context(date=purchase_order.date_order)

Referred: https://stackoverflow.com/a/44983955

However, if on a certain date, the rate is not defined, it will take the rate as 1. Is it possible to make it take the next nearest dates rate? How do we do that.

Thanks.    

You analyse the file /addons/base/res/res_currency.py

The field rate is defined as:

    rate = fields.Float(compute='_compute_current_rate', string='Current Rate', digits=(12, 6),
                        help='The rate of the currency to the currency of rate 1.')

and computed by method (bellow), which depends on 'rate_ids.rate':

    @api.multi
    @api.depends('rate_ids.rate')
    def _compute_current_rate(self):
        date = self._context.get('date') or fields.Date.today()
        company_id = self._context.get('company_id') or self.env['res.users']._get_company().id
        # the subquery selects the last rate before 'date' for the given currency/company
        query = """SELECT c.id, (SELECT r.rate FROM res_currency_rate r
                                  WHERE r.currency_id = c.id AND r.name <= %s
                                    AND (r.company_id IS NULL OR r.company_id = %s)
                               ORDER BY r.company_id, r.name DESC
                                  LIMIT 1) AS rate
                   FROM res_currency c
                   WHERE c.id IN %s"""
        self._cr.execute(query, (date, company_id, tuple(self.ids)))
        currency_rates = dict(self._cr.fetchall())
        for currency in self:
            currency.rate = currency_rates.get(currency.id) or 1.0

Conclusion, if rate_ids are defined correctly, method should return the last rate.

You find in file, additionaly this code (used to calculate the date of currency) :

    @api.multi
    @api.depends('rate_ids.name')
    def _compute_date(self):
        for currency in self:
            currency.date = currency.rate_ids[:1].name

Conclusion: rate_ids.name may be used to search rate on a specified date (if exist)