Insert records into the database.

        - `model_instances`: any iterable containing instances of a single model class.
        - `batch_size`: number of records to send per chunk (use a lower number if your records are very large).

def insert(self, model_instances, batch_size=1000):
        '''
        Insert records into the database.

        - `model_instances`: any iterable containing instances of a single model class.
        - `batch_size`: number of records to send per chunk (use a lower number if your records are very large).
        '''
        from six import next
        from io import BytesIO
        i = iter(model_instances)
        try:
            first_instance = next(i)
        except StopIteration:
            return  # model_instances is empty
        model_class = first_instance.__class__

        if first_instance.is_read_only() or first_instance.is_system_model():
            raise DatabaseException("You can't insert into read only and system tables")

        fields_list = ','.join(
            ['`%s`' % name for name in first_instance.fields(writable=True)])

        def gen():
            buf = BytesIO()
            query = 'INSERT INTO $table (%s) FORMAT TabSeparated\n' % fields_list
            buf.write(self._substitute(query, model_class).encode('utf-8'))
            first_instance.set_database(self)
            buf.write(first_instance.to_tsv(include_readonly=False).encode('utf-8'))
            buf.write('\n'.encode('utf-8'))
            # Collect lines in batches of batch_size
            lines = 2
            for instance in i:
                instance.set_database(self)
                buf.write(instance.to_tsv(include_readonly=False).encode('utf-8'))
                buf.write('\n'.encode('utf-8'))
                lines += 1
                if lines >= batch_size:
                    # Return the current batch of lines
                    yield buf.getvalue()
                    # Start a new batch
                    buf = BytesIO()
                    lines = 0
            # Return any remaining lines in partial batch
            if lines:
                yield buf.getvalue()
        self._send(gen())