diff --git a/Gemfile b/Gemfile
index 32cc6fb..fffa312 100644
--- a/Gemfile
+++ b/Gemfile
@@ -2,7 +2,7 @@ source 'https://rubygems.org'
 
 gemspec
 
-logstash_path = ENV["LOGSTASH_PATH"] || "../../logstash"
+logstash_path = ENV["LOGSTASH_PATH"] || "."
 use_logstash_source = ENV["LOGSTASH_SOURCE"] && ENV["LOGSTASH_SOURCE"].to_s == "1"
 
 if Dir.exist?(logstash_path) && use_logstash_source
diff --git a/lib/logstash/outputs/bson/big_decimal.rb b/lib/logstash/outputs/bson/big_decimal.rb
index 3b5d4ef..fd06f9e 100644
--- a/lib/logstash/outputs/bson/big_decimal.rb
+++ b/lib/logstash/outputs/bson/big_decimal.rb
@@ -50,7 +50,7 @@ def from_bson(bson)
       private
 
       def from_bson_double(double)
-        new(double.unpack(PACK).first.to_s)
+        BigDecimal(double.unpack(PACK).first.to_s)
       end
     end
 
diff --git a/lib/logstash/outputs/mongodb.rb b/lib/logstash/outputs/mongodb.rb
index 0b88c68..adc3e88 100644
--- a/lib/logstash/outputs/mongodb.rb
+++ b/lib/logstash/outputs/mongodb.rb
@@ -34,6 +34,12 @@ class LogStash::Outputs::Mongodb < LogStash::Outputs::Base
   # "_id" field in the event.
   config :generateId, :validate => :boolean, :default => false
 
+  # The field that will be used for the _id field
+  # This can be for example the ID column of a SQL table when using JDBC.
+  config :idField, :validate => :string, :required => false
+
+  # Upsert documents flag, set to true to use replace_one instead of insert_one.
+  config :upsert, :validate => :boolean, :default => false
 
   # Bulk insert flag, set to true to allow bulk insertion, else it will insert events one by one.
   config :bulk, :validate => :boolean, :default => false
@@ -65,7 +71,14 @@ def register
         @@mutex.synchronize do
           @documents.each do |collection, values|
             if values.length > 0
-              @db[collection].insert_many(values)
+              if @upsert
+                bulk_operations = values.map do |doc|
+                  { replace_one: { filter: { _id: doc["_id"] }, replacement: doc, upsert: true } }
+                end
+                @db[collection].bulk_write(bulk_operations)
+              else
+                @db[collection].insert_many(values)
+              end
               @documents.delete(collection)
             end
           end
@@ -94,6 +107,17 @@ def receive(event)
         document["_id"] = BSON::ObjectId.new
       end
 
+      if @idField
+        field_name = event.sprintf(@idField)
+        if event.include?(field_name) && !event.get(field_name).nil?
+          document["_id"] = event.get(field_name)
+        else
+          @logger.warn("Cannot set MongoDB document `_id` field because it does not exist in the event", :event => event)
+          document["_id"] = BSON::ObjectId.new
+        end
+      end
+
+
       if @bulk
         collection = event.sprintf(@collection)
         @@mutex.synchronize do
@@ -103,12 +127,23 @@ def receive(event)
           @documents[collection].push(document)
 
           if(@documents[collection].length >= @bulk_size)
-            @db[collection].insert_many(@documents[collection])
+            if @upsert && document.key?("_id")
+                bulk_operations = @documents[collection].map do |doc|
+                  { replace_one: { filter: { _id: doc["_id"] }, replacement: doc, upsert: true } }
+                end
+                @db[collection].bulk_write(bulk_operations)
+            else
+                @db[collection].insert_many(@documents[collection])
+            end
             @documents.delete(collection)
           end
         end
       else
-        @db[event.sprintf(@collection)].insert_one(document)
+        if @upsert && document.key?("_id")
+          @db[event.sprintf(@collection)].replace_one({ _id: document["_id"] }, document, { upsert: true })
+        else
+          @db[event.sprintf(@collection)].insert_one(document)
+        end
       end
     rescue => e
       if e.message =~ /^E11000/
diff --git a/logstash-output-mongodb.gemspec b/logstash-output-mongodb.gemspec
index b2cd699..0f1f3af 100644
--- a/logstash-output-mongodb.gemspec
+++ b/logstash-output-mongodb.gemspec
@@ -24,5 +24,4 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency 'mongo', '~> 2.6'
 
   s.add_development_dependency 'logstash-devutils'
-end
-
+end
\ No newline at end of file
diff --git a/spec/bson/big_decimal_spec.rb b/spec/bson/big_decimal_spec.rb
index b515e9d..3abfbd4 100644
--- a/spec/bson/big_decimal_spec.rb
+++ b/spec/bson/big_decimal_spec.rb
@@ -23,7 +23,7 @@
   describe "class methods" do
     it "builds a new BigDecimal from BSON" do
       decoded = described_class.from_bson(4321.1234.to_bson)
-      expect(decoded).to eql(BigDecimal.new(a_number))
+      expect(decoded).to eql(BigDecimal(a_number))
     end
   end
 end
diff --git a/spec/integration/mongodb_spec.rb b/spec/integration/mongodb_spec.rb
index 690e8fa..bdaf1a6 100644
--- a/spec/integration/mongodb_spec.rb
+++ b/spec/integration/mongodb_spec.rb
@@ -18,7 +18,7 @@
     subject { LogStash::Outputs::Mongodb.new(config) }
 
     let(:properties) { { "message" => "This is a message!",
-                         "uuid" => uuid, "number" => BigDecimal.new("4321.1234"),
+                         "uuid" => uuid, "number" => BigDecimal("4321.1234"),
                          "utf8" => "żółć", "int" => 42,
                          "arry" => [42, "string", 4321.1234]} }
     let(:event)      { LogStash::Event.new(properties) }
diff --git a/spec/outputs/mongodb_spec.rb b/spec/outputs/mongodb_spec.rb
index 8d3decb..3cecfd4 100644
--- a/spec/outputs/mongodb_spec.rb
+++ b/spec/outputs/mongodb_spec.rb
@@ -44,7 +44,7 @@
       let(:properties) {{
         "message" => "This is a message!",
         "uuid" => SecureRandom.uuid,
-        "number" => BigDecimal.new("4321.1234"),
+        "number" => BigDecimal("4321.1234"),
         "utf8" => "żółć"
       }}