Attempt to maintain encoding for arrays of strings with PG

I still think that this is something that should be handled in the pg
gem, but it's not going to end up happening there so we'll do it here
instead. Once we bump to pg 0.19 we can pass the encoding to the
`encode` method instead.

This issue occurs because C has no concept of encoding (or strings,
really). The bytes that we pass here when sending the value to the
database will always be interpreted as whatever encoding the connection
is currently configured to use. That means that roundtripping to the
database will lose no information

However, after assigning we round trip through our type system without
hitting the database. The only way that we can do the "correct" thin
here would be to actually give a reference to the connection to the
array type and have it check the current value of the connection's
encoding -- which I'm strongly opposed to. We could also pass in the
encoding when it's constructed, but since that can change independently
of the type I'm not a huge fan of that either.

This feels like a reasonable middle ground, where if we have an array of
strings we simply use the encoding of the string we're given.

Fixes #26326.

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   PostgreSQL array columns will now respect the encoding of strings contained
+    in the array.
+
+    Fixes #26326.
+
+    *Sean Griffin*
+
 *   Inverse association instances will now be set before `after_find` or
     `after_initialize` callbacks are run.
 

activerecord/lib/active_record/connection_adapters/postgresql/oid/array.rb

@@ -33,7 +33,11 @@ module ActiveRecord
 
           def serialize(value)
             if value.is_a?(::Array)
-              @pg_encoder.encode(type_cast_array(value, :serialize))
+              result = @pg_encoder.encode(type_cast_array(value, :serialize))
+              if encoding = determine_encoding_of_strings(value)
+                result.encode!(encoding)
+              end
+              result
             else
               super
             end
@@ -63,6 +67,13 @@ module ActiveRecord
                 @subtype.public_send(method, value)
               end
             end
+
+            def determine_encoding_of_strings(value)
+              case value
+              when ::Array then determine_encoding_of_strings(value.first)
+              when ::String then value.encoding
+              end
+            end
         end
       end
     end

activerecord/test/cases/adapters/postgresql/array_test.rb

@@ -311,6 +311,12 @@ class PostgresqlArrayTest < ActiveRecord::PostgreSQLTestCase
     assert_equal ["has already been taken"], e2.errors[:tags], "Should have uniqueness message for tags"
   end
 
+  def test_encoding_arrays_of_utf8_strings
+    string_with_utf8 = "nový"
+    assert_equal [string_with_utf8], @type.deserialize(@type.serialize([string_with_utf8]))
+    assert_equal [[string_with_utf8]], @type.deserialize(@type.serialize([[string_with_utf8]]))
+  end
+
   private
     def assert_cycle(field, array)
       # test creation