Warn when passing :exact option that has no effect

lib/capybara/node/finders.rb

@@ -31,7 +31,7 @@ module Capybara
       def find(*args)
         query = Capybara::Queries::SelectorQuery.new(*args)
         synchronize(query.wait) do
-          if query.match == :smart or query.match == :prefer_exact
+          if (query.match == :smart or query.match == :prefer_exact) and query.supports_exact?
             result = query.resolve_for(self, true)
             result = query.resolve_for(self, false) if result.empty? && !query.exact?
           else

lib/capybara/queries/selector_query.rb

@@ -27,6 +27,9 @@ module Capybara
         end
 
         @expression = @selector.call(@locator)
+
+        warn_exact_usage(@expression.to_s)
+
         assert_valid_keys
       end
 
@@ -79,6 +82,7 @@ module Capybara
       end
 
       def exact?
+        return false if !supports_exact?
         if options.has_key?(:exact)
           @options[:exact]
         else
@@ -125,6 +129,11 @@ module Capybara
         end
       end
 
+      # @api private
+      def supports_exact?
+        @expression.respond_to? :to_xpath
+      end
+
       private
 
       def valid_keys
@@ -150,6 +159,13 @@ module Capybara
           raise ArgumentError, "invalid option #{match.inspect} for :match, should be one of #{VALID_MATCH.map(&:inspect).join(", ")}"
         end
       end
+
+      def warn_exact_usage(expression)
+        if options.has_key?(:exact) && !supports_exact?
+          warn "The :exact option only has an effect on queries using the XPath#is method. Using it with the query \"#{expression}\" has no effect."
+        end
+        expression
+      end
     end
   end
 end

lib/capybara/spec/session/find_spec.rb

@@ -235,6 +235,12 @@ Capybara::SpecHelper.spec '#find' do
       Capybara.exact = false
       @session.find(:xpath, XPath.descendant(:input)[XPath.attr(:id).is("est_fiel")])
     end
+
+    it "warns when the option has no effect" do
+      expect_any_instance_of(Kernel).to receive(:warn).
+        with('The :exact option only has an effect on queries using the XPath#is method. Using it with the query "#test_field" has no effect.')
+      @session.find(:css, '#test_field', exact: true)
+    end
   end
 
   context "with :match option" do
@@ -292,7 +298,7 @@ Capybara::SpecHelper.spec '#find' do
         end
         it "raises an error if there is no match" do
           expect do
-            @session.find(:css, ".does-not-exist", :match => :smart, :exact => false)
+            @session.find(:xpath, XPath.descendant[XPath.attr(:class).is("does-not-exist")], :match => :smart, :exact => false)
           end.to raise_error(Capybara::ElementNotFound)
         end
       end
@@ -319,7 +325,7 @@ Capybara::SpecHelper.spec '#find' do
         end
         it "raises an error if there is no match" do
           expect do
-            @session.find(:css, ".does-not-exist", :match => :smart, :exact => true)
+            @session.find(:xpath, XPath.descendant[XPath.attr(:class).is("does-not-exist")], :match => :smart, :exact => true)
           end.to raise_error(Capybara::ElementNotFound)
         end
       end
@@ -345,7 +351,7 @@ Capybara::SpecHelper.spec '#find' do
         end
         it "raises an error if there is no match" do
           expect do
-            @session.find(:css, ".does-not-exist", :match => :prefer_exact, :exact => false)
+            @session.find(:xpath, XPath.descendant[XPath.attr(:class).is("does-not-exist")], :match => :prefer_exact, :exact => false)
           end.to raise_error(Capybara::ElementNotFound)
         end
       end
@@ -371,7 +377,7 @@ Capybara::SpecHelper.spec '#find' do
         end
         it "raises an error if there is no match" do
           expect do
-            @session.find(:css, ".does-not-exist", :match => :prefer_exact, :exact => true)
+            @session.find(:xpath, XPath.descendant[XPath.attr(:class).is("does-not-exist")], :match => :prefer_exact, :exact => true)
           end.to raise_error(Capybara::ElementNotFound)
         end
       end