Add 'USER_LOGIN_FIELDS_UNIQUE_CHECK_ENABLED' setting (#208)

docs/detailed_configuration/settings_fields.j2

@@ -6,15 +6,22 @@
 
 '{{ f.name }}'
 ~{{ '~' * f.name|length }}~
+{% if f.type_signature %}
+{% if f.type_signature|attr("__class__")|attr("__name__") == "type" %}
+* Type: ``{{ f.type_signature.__name__ }}``
+{% else %}
+* Type: ``{{ f.type_signature|pprint }}``
+{% endif %}
+{% endif %}
 {% if (f.default|pprint).split('\n')|length > 1 %}
-Default:
+* Default:
 
 .. code:: python
 
     {{ f.default|pprint|indent(width=4) }}
 
 {% else %}
-Default: ``{{ f.default|pprint }}``
+* Default: ``{{ f.default|pprint }}``
 {% endif %}
 {% if f.help %}
 {{ f.help }}

rest_registration/checks.py

@@ -16,6 +16,7 @@
 from rest_registration.utils.users import (
     get_user_email_field_name,
     get_user_login_field_names,
+    get_user_setting,
     is_model_field_unique
 )
 
@@ -249,7 +250,10 @@ def drf_compatible_django_auth_backend_check() -> bool:
     ErrorCode.LOGIN_FIELDS_NOT_UNIQUE,
 )
 def login_fields_unique_check() -> bool:
-    return implies(_is_auth_installed(), _are_login_fields_unique)
+    return implies(
+        _is_auth_installed() and get_user_setting('LOGIN_FIELDS_UNIQUE_CHECK_ENABLED'),
+        _are_login_fields_unique,
+    )
 
 
 def _are_login_fields_unique() -> bool:

rest_registration/settings_fields.py

@@ -1,10 +1,12 @@
 import datetime
 from collections import OrderedDict, namedtuple
 from textwrap import dedent
+from typing import List, Union
 
 _Field = namedtuple('_Field', [
     'name',
     'default',
+    'type_signature',
     'help',
     'import_string',
 ])
@@ -15,16 +17,45 @@ class Field(_Field):
     def __new__(
             cls, name, *,
             default=None,
+            type_signature=None,
             help=None,  # pylint: disable=redefined-builtin
             import_string=False):
         return super().__new__(
             cls, name=name, default=default,
+            type_signature=type_signature,
             help=help, import_string=import_string)
 
 
 USER_SETTINGS_FIELDS = [
     Field(
         'USER_LOGIN_FIELDS',
+        type_signature=Union[List[str], None],
+        help=dedent("""\
+            Setting that defines the list of fields which can be treated as
+            "login" fields. An example of such fields would be ``username``, ``email``.
+
+            If not set / set to ``None``, it will fall back to one-element list
+            containing ``user_class.USERNAME_FIELD``,
+
+            This setting is used by default :ref:`login-authenticator-setting`
+            implementation and default
+            :ref:`send-reset-password-link-user-finder-setting` implementation.
+
+            To ensure integrity, Django-REST-Registration will check if each of
+            the listed fields is unique. If you want to disable that check,
+            please refer to :ref:`user-login-fields-unique-check-enabled-setting`.
+            """)
+    ),
+    Field(
+        'USER_LOGIN_FIELDS_UNIQUE_CHECK_ENABLED',
+        type_signature=bool,
+        default=True,
+        help=dedent("""\
+            If enabled, each of the fields listed in :ref:`user-login-fields-setting`
+            will be checked if it is unique.
+
+            **Disable this setting only when you know what you're doing!**
+            """)
     ),
     Field(
         'USER_HIDDEN_FIELDS',

tests/unit_tests/test_checks.py

@@ -295,6 +295,15 @@ def test_when_one_non_unique_login_field_then_check_fails():
     ])
 
 
+@override_rest_registration_settings({
+    'USER_LOGIN_FIELDS': ['email'],
+    'USER_LOGIN_FIELDS_UNIQUE_CHECK_ENABLED': False,
+})
+def test_ok_when_one_non_unique_login_field_but_check_disabled():
+    errors = simulate_checks()
+    assert_error_codes_match(errors, [])
+
+
 @override_rest_registration_settings({
     'USER_LOGIN_FIELDS': ['username', 'email'],
 })