MONGOID-5168 Replace field[0] == ?$ with field.start_with?('$') -- easier to read and 30%+ faster (#5052)

Co-authored-by: shields <[email protected]>

Author: johnnyshields

lib/mongoid/criteria/queryable/selectable.rb

@@ -597,7 +597,7 @@ def not(*criteria)
               end
               _mongoid_expand_keys(new_s).each do |k, v|
                 k = k.to_s
-                if c.selector[k] || k[0] == ?$
+                if c.selector[k] || k.start_with?('$')
                   c = c.send(:__multi__, [{'$nor' => [{k => v}]}], '$and')
                 else
                   if v.is_a?(Hash)
@@ -890,7 +890,7 @@ def expr_query(criterion)
           clone.tap do |query|
             normalized.each do |field, value|
               field_s = field.to_s
-              if field_s[0] == ?$
+              if field_s.start_with?('$')
                 # Query expression-level operator, like $and or $where
                 query.add_operator_expression(field_s, value)
               else

lib/mongoid/criteria/queryable/storable.rb

@@ -38,7 +38,7 @@ def add_field_expression(field, value)
             raise ArgumentError, "Field must be a string: #{field}"
           end
 
-          if field[0] == ?$
+          if field.start_with?('$')
             raise ArgumentError, "Field cannot be an operator (i.e. begin with $): #{field}"
           end
 
@@ -48,7 +48,7 @@ def add_field_expression(field, value)
             if value.is_a?(Hash) && selector[field].is_a?(Hash) &&
               value.keys.all? { |key|
                 key_s = key.to_s
-                key_s[0] == ?$ && !selector[field].key?(key_s)
+                key_s.start_with?('$') && !selector[field].key?(key_s)
               }
             then
               # Multiple operators can be combined on the same field by
@@ -185,7 +185,7 @@ def add_operator_expression(operator, op_expr)
             raise ArgumentError, "Operator must be a string: #{operator}"
           end
 
-          unless operator[0] == ?$
+          unless operator.start_with?('$')
             raise ArgumentError, "Operator must begin with $: #{operator}"
           end
 
@@ -220,7 +220,7 @@ def add_one_expression(field, value)
             raise ArgumentError, "Field must be a string: #{field}"
           end
 
-          if field[0] == ?$
+          if field.start_with?('$')
             add_operator_expression(field, value)
           else
             add_field_expression(field, value)