def acquisition_source(self, key, value):
    """Populate the ``acquisition_source`` key."""
    def _get_datetime(value):
        d_value = force_single_element(value.get('d', ''))
        if d_value:
            try:
                date = PartialDate.loads(d_value)
            except ValueError:
                return d_value
            else:
                datetime_ = datetime(year=date.year, month=date.month, day=date.day)
                return datetime_.isoformat()

    internal_uid, orcid, source = None, None, None

    a_values = force_list(value.get('a'))
    for a_value in a_values:
        if IS_INTERNAL_UID.match(a_value):
            if a_value.startswith('inspire:uid:'):
                internal_uid = int(a_value[12:])
            else:
                internal_uid = int(a_value)
        elif IS_ORCID.match(a_value):
            if a_value.startswith('orcid:'):
                orcid = a_value[6:]
            else:
                orcid = a_value
        else:
            source = a_value

    c_value = force_single_element(value.get('c', ''))
    normalized_c_value = c_value.lower()

    if normalized_c_value == 'batchupload':
        method = 'batchuploader'
    elif normalized_c_value == 'submission':
        method = 'submitter'
    else:
        method = normalized_c_value

    return {
        'datetime': _get_datetime(value),
        'email': value.get('b'),
        'internal_uid': internal_uid,
        'method': method,
        'orcid': orcid,
        'source': source,
        'submission_number': value.get('e'),
    }
Populate the ``acquisition_source`` key.