Make supported_collations lookup dependent on the collencoding value

lib/database_collation.rb

@@ -54,9 +54,14 @@ def postgresql_version
   end
 
   def supported_collations
-    @supported_collations ||= connection.
-      execute(%q(SELECT collname FROM pg_collation;)).
-        map { |row| row['collname'] }
+    sql = <<-EOF.strip_heredoc.squish
+    SELECT collname FROM pg_collation
+    WHERE collencoding = '-1'
+    OR collencoding = '#{ database_encoding }';
+    EOF
+
+    @supported_collations ||=
+      connection.execute(sql).map { |row| row['collname'] }
   end
 
   def database_encoding

spec/lib/database_collation_spec.rb

@@ -45,11 +45,21 @@
       expect(database.supports?('es')).to be false
     end
 
-    it 'supports collation if the collation exists' do
+    it 'does not support collation if the collation has a different encoding' do
+      database = DatabaseCollation.new(mock_connection)
+      expect(database.supports?('en_GB.utf8')).to be false
+    end
+
+    it 'supports collation if the collation exists for current encoding' do
       database = DatabaseCollation.new(mock_connection)
       expect(database.supports?('en_GB')).to be true
     end
 
+    it 'supports installed collations with "-1" (universal) encoding' do
+      database = DatabaseCollation.new(mock_connection)
+      expect(database.supports?('default')).to be true
+    end
+
   end
 
 end
@@ -64,18 +74,33 @@ def mock_connection(connection_double_opts = {})
   connection = double('ActiveRecord::FakeConnection', connection_double_opts)
 
   installed_collations = [
-    { "collname" => "default" },
-    { "collname" => "C" },
-    { "collname" => "POSIX" },
-    { "collname" => "C.UTF-8" },
-    { "collname" => "en_GB" },
-    { "collname" => "en_GB.utf8" }
+    { "collname" => "default", "collencoding" => "-1"},
+    { "collname" => "C", "collencoding" => "-1" },
+    { "collname" => "POSIX", "collencoding" => "-1" },
+    { "collname" => "C.UTF-8", "collencoding" => "6" },
+    { "collname" => "en_GB", "collencoding" => "8" },
+    { "collname" => "en_GB.utf8", "collencoding" => "6" }
   ]
 
+  allow(connection).to receive(:current_database).and_return("alaveteli_test")
+
+  allow(connection).
+    to receive(:execute).
+      with("SELECT encoding FROM pg_database WHERE datname = " \
+           "'alaveteli_test';").
+      and_return([{ "encoding" => "8" }])
+
+  # Simulate SQL filtering of returned collations
   allow(connection).
     to receive(:execute).
-      with(%q(SELECT collname FROM pg_collation;)).
-        and_return(installed_collations)
+      with("SELECT collname FROM pg_collation " \
+           "WHERE collencoding = '-1' OR collencoding = '8';").
+        and_return(filter_collations(installed_collations, %w(-1 8)))
 
   connection
 end
+
+def filter_collations(collations, encodings)
+  collations.
+    select { |collation| encodings.include?(collation["collencoding"]) }
+end