Modify behaviour of `AM::Errors#to_h`:

- `AM::Error#to_h` was kind of broken before and would return in the
  hash values a single error message.

  ```ruby
    person = Person.new
    person.errors.add(:name, "cannot be blank")
    person.errors.add(:name, "too long")

    puts person.errors.to_h # {name: 'too long'}
  ```

  Since an attribute can have different errors, the previous behavior
  didn't make much sense.

  Now, `ActiveModel::Errors#to_hash` correctly returns an array of
  error messages containing all the errors for an attribute.
  However, one can easily be surprised by this change, so let's
  deprecated it first.

2 files changed, 9 insertions, 4 deletions

diff --git a/activemodel/lib/active_model/errors.rb b/activemodel/lib/active_model/errors.rb
index e7405fb586..480f24183a 100644
--- a/activemodel/lib/active_model/errors.rb
+++ b/activemodel/lib/active_model/errors.rb
@@ -304,9 +304,13 @@ module ActiveModel
     end
 
     def to_h
-      deprecation_rename_warning(:to_h, :to_hash)
+      ActiveSupport::Deprecation.warn(<<~EOM)
+        ActiveModel::Errors#to_h is deprecated and will be removed in Rails 6.2
+        Please use `ActiveModel::Errors.to_hash` instead. The values in the hash
+        returned by `ActiveModel::Erors.to_hash` is an array of error messages.
+      EOM
 
-      to_hash
+      to_hash.transform_values { |values| values.last }
     end
 
     def messages
diff --git a/activemodel/test/cases/errors_test.rb b/activemodel/test/cases/errors_test.rb
index 79ccfe0c13..a6cd1da717 100644
--- a/activemodel/test/cases/errors_test.rb
+++ b/activemodel/test/cases/errors_test.rb
@@ -477,10 +477,11 @@ class ErrorsTest < ActiveModel::TestCase
   test "to_h is deprecated" do
     person = Person.new
     person.errors.add(:name, "cannot be blank")
+    person.errors.add(:name, "too long")
 
-    expected_deprecation = "ActiveModel::Errors#to_h is deprecated. Please call #to_hash instead."
+    expected_deprecation = "ActiveModel::Errors#to_h is deprecated"
     assert_deprecated(expected_deprecation) do
-      assert_equal({ name: ["cannot be blank"] }, person.errors.to_h)
+      assert_equal({ name: "too long" }, person.errors.to_h)
     end
   end