MONGOID-6016 Short-circuit database_field_name for keys without "."

lib/mongoid/fields.rb

@@ -412,6 +412,13 @@ def database_field_name(name, relations, aliased_fields, aliased_associations)
         return '' unless name.present?
 
         key = name.to_s
+
+        # A key without a "." refers to an immediate field, so it resolves to
+        # its alias (or itself) with no splitting. Short-circuiting here avoids
+        # the split and dup on every read, which is a hot path. This mirrors the
+        # same optimization already applied to the matcher.
+        return aliased_fields[key]&.dup || key unless key.include?('.')
+
         segment, remaining = key.split('.', 2)
 
         # Don't get the alias for the field when a belongs_to association

spec/mongoid/fields_spec.rb

@@ -1814,6 +1814,14 @@ class DiscriminatorChild2 < DiscriminatorParent
       it { is_expected.to eq '' }
     end
 
+    context 'when the returned name is mutated' do
+      it 'does not corrupt the aliased field map' do
+        Person.database_field_name('test') << '_mutated'
+
+        expect(Person.database_field_name('test')).to eq('t')
+      end
+    end
+
     context 'given a String' do
       subject { Person.database_field_name(key.to_s) }