Fixed #16192 -- Made unique error messages in ModelForm customizable.

Overriding the error messages now works for both unique fields, unique_together
and unique_for_date.

This patch changed the overriding logic to allow customizing NON_FIELD_ERRORS
since previously only fields' errors were customizable.

Refs #20199.

Thanks leahculver for the suggestion.

6 files changed, 48 insertions, 5 deletions

diff --git a/docs/ref/exceptions.txt b/docs/ref/exceptions.txt
index 956ee5a188..c84dd87ed3 100644
--- a/docs/ref/exceptions.txt
+++ b/docs/ref/exceptions.txt
@@ -124,6 +124,15 @@ ValidationError
     :ref:`Model Field Validation <validating-objects>` and the
     :doc:`Validator Reference </ref/validators>`.
 
+NON_FIELD_ERRORS
+~~~~~~~~~~~~~~~~
+.. data:: NON_FIELD_ERRORS
+
+``ValidationError``\s that don't belong to a particular field in a form
+or model are classified as ``NON_FIELD_ERRORS``. This constant is used
+as a key in dictonaries that otherwise map fields to their respective
+list of errors.
+
 .. currentmodule:: django.core.urlresolvers
 
 URL Resolver exceptions
diff --git a/docs/ref/models/fields.txt b/docs/ref/models/fields.txt
index 22fadd26c3..c6a5311760 100644
--- a/docs/ref/models/fields.txt
+++ b/docs/ref/models/fields.txt
@@ -233,8 +233,12 @@ field will raise. Pass in a dictionary with keys matching the error messages you
 want to override.
 
 Error message keys include ``null``, ``blank``, ``invalid``, ``invalid_choice``,
-and ``unique``. Additional error message keys are specified for each field in
-the `Field types`_ section below.
+``unique``, and ``unique_for_date``. Additional error message keys are
+specified for each field in the `Field types`_ section below.
+
+.. versionadded:: 1.7
+
+The ``unique_for_date`` error message key was added.
 
 ``help_text``
 -------------
diff --git a/docs/ref/models/instances.txt b/docs/ref/models/instances.txt
index 6bc6917a9e..2177bc6a59 100644
--- a/docs/ref/models/instances.txt
+++ b/docs/ref/models/instances.txt
@@ -151,8 +151,8 @@ access to more than a single field::
 
 Any :exc:`~django.core.exceptions.ValidationError` exceptions raised by
 ``Model.clean()`` will be stored in a special key error dictionary key,
-``NON_FIELD_ERRORS``, that is used for errors that are tied to the entire model
-instead of to a specific field::
+:data:`~django.core.exceptions.NON_FIELD_ERRORS`, that is used for errors
+that are tied to the entire model instead of to a specific field::
 
     from django.core.exceptions import ValidationError, NON_FIELD_ERRORS
     try:
diff --git a/docs/ref/models/options.txt b/docs/ref/models/options.txt
index ba07dfaeb9..ae2d7def0f 100644
--- a/docs/ref/models/options.txt
+++ b/docs/ref/models/options.txt
@@ -321,6 +321,11 @@ Django quotes column and table names behind the scenes.
     :class:`~django.db.models.ManyToManyField`, try using a signal or
     an explicit :attr:`through <ManyToManyField.through>` model.
 
+    .. versionchanged:: 1.7
+
+    The ``ValidationError`` raised during model validation when the
+    constraint is violated has the ``unique_together`` error code.
+
 ``index_together``
 ------------------
 
diff --git a/docs/releases/1.7.txt b/docs/releases/1.7.txt
index a51c3dc981..f9d3a7e795 100644
--- a/docs/releases/1.7.txt
+++ b/docs/releases/1.7.txt
@@ -484,6 +484,14 @@ Forms
   that maps fields to their original errors, complete with all metadata
   (error code and params), the latter returns the errors serialized as json.
 
+* It's now possible to customize the error messages for ``ModelForm``’s
+  ``unique``, ``unique_for_date``, and ``unique_together`` constraints.
+  In order to support ``unique_together`` or any other ``NON_FIELD_ERROR``,
+  ``ModelForm`` now looks for the ``NON_FIELD_ERROR`` key in the
+  ``error_messages`` dictionary of the ``ModelForm``’s inner ``Meta`` class.
+  See :ref:`considerations regarding model's error_messages
+  <considerations-regarding-model-errormessages>` for more details.
+
 Internationalization
 ^^^^^^^^^^^^^^^^^^^^
 
diff --git a/docs/topics/forms/modelforms.txt b/docs/topics/forms/modelforms.txt
index e98fcb0344..1cac2d6e7e 100644
--- a/docs/topics/forms/modelforms.txt
+++ b/docs/topics/forms/modelforms.txt
@@ -259,7 +259,9 @@ The model's ``clean()`` method will be called before any uniqueness checks are
 made. See :ref:`Validating objects <validating-objects>` for more information
 on the model's ``clean()`` hook.
 
-Considerations regarding fields' ``error_messages``
+.. _considerations-regarding-model-errormessages:
+
+Considerations regarding model's ``error_messages``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Error messages defined at the
@@ -267,12 +269,27 @@ Error messages defined at the
 :ref:`form Meta <modelforms-overriding-default-fields>` level always take
 precedence over the error messages defined at the
 :attr:`model field <django.db.models.Field.error_messages>` level.
+
 Error messages  defined on :attr:`model fields
 <django.db.models.Field.error_messages>` are only used when the
 ``ValidationError`` is raised during the :ref:`model validation
 <validating-objects>` step and no corresponding error messages are defined at
 the form level.
 
+.. versionadded:: 1.7
+
+You can override the error messages from ``NON_FIELD_ERRORS`` raised by model
+validation by adding the :data:`~django.core.exceptions.NON_FIELD_ERRORS` key
+to the ``error_messages`` dictionary of the ``ModelForm``’s inner ``Meta`` class::
+
+    class ArticleForm(ModelForm):
+        class Meta:
+            error_messages = {
+                NON_FIELD_ERRORS: {
+                    'unique_together': "%(model_name)s's %(field_labels)s are not unique.",
+                }
+            }
+
 The ``save()`` method
 ---------------------