Use strong consistency for map/reduce operations. [ fix #2241 ]

CHANGELOG.md

@@ -10,6 +10,9 @@ For instructions on upgrading to newer versions, visit
 * \#2242 Fix eager loading not to load all documents when calling first or
   last.
 
+* \#2241 Map/reduce operations now always use strong consistency since they
+  have the potential to write to collections, most of the time.
+
 * \#2224 `Model#inc` now accepts `BigDecimal` values.
 
 * \#2212 Ensure errors are cleared after a save with `validate: false` in all

lib/mongoid/contextual/map_reduce.rb

@@ -266,7 +266,7 @@ def output_collection
       # @since 3.0.0
       def results
         raise Errors::NoMapReduceOutput.new(command) unless command[:out]
-        @results ||= session.command(command)
+        @results ||= session.with(consistency: :strong).command(command)
       end
     end
   end

spec/config/mongoid.yml

@@ -5,7 +5,7 @@ test:
       hosts:
         - <%=ENV["MONGOID_SPEC_HOST"]%>:<%=ENV["MONGOID_SPEC_PORT"]%>
       options:
-        consistency: :strong
+        consistency: :eventual
     mongohq_single:
       database: <%=ENV["MONGOHQ_SINGLE_NAME"]%>
       username: <%=ENV["MONGOHQ_SINGLE_USER"]%>

spec/mongoid/config_spec.rb

@@ -164,7 +164,7 @@
             end
 
             it "sets the consistency option" do
-              options["consistency"].should eq(:strong)
+              options["consistency"].should eq(:eventual)
             end
           end
         end

spec/mongoid/sessions_spec.rb

@@ -516,13 +516,28 @@
             create(name: "Tool", likes: 100)
         end
 
-        let(:results) do
-          Band.with(database: "mongoid_test_alt").
-            map_reduce(map, reduce).out(inline: 1)
+        context "when outputting in memory" do
+
+          let(:results) do
+            Band.with(database: "mongoid_test_alt").
+              map_reduce(map, reduce).out(inline: 1)
+          end
+
+          it "executes the map/reduce on the correct database" do
+            results.first["value"].should eq({ "likes" => 200 })
+          end
         end
 
-        it "executes the map/reduce on the correct database" do
-          results.first["value"].should eq({ "likes" => 200 })
+        context "when outputting to a collection" do
+
+          let(:results) do
+            Band.with(database: "mongoid_test_alt").
+              map_reduce(map, reduce).out(replace: "bands_output")
+          end
+
+          it "executes the map/reduce on the correct database" do
+            results.first["value"].should eq({ "likes" => 200 })
+          end
         end
       end
     end