Remove unnecessary object allocation in Model#exists?

Use a private constant instead.

Update CHANGELOG for previous commit.

CHANGELOG

@@ -1,5 +1,7 @@
 === master
 
+* Speed up validates_unique in validation_helpers plugin by using empty? instead of count == 0 (numbata) (#2122)
+
 * Speed up regexp matches in sqlite adapter on Ruby 2.4+ (jeremyevans)
 
 * Add sqlite adapter :regexp_function_cache option for specifying the cache object to use (paddor, jeremyevans) (#2116)

lib/sequel/model/base.rb

@@ -1244,18 +1244,21 @@ def errors
         @errors ||= errors_class.new
       end 
 
+      EXISTS_SELECT_ = SQL::AliasedExpression.new(1, :one)
+      private_constant :EXISTS_SELECT_
+
       # Returns true when current instance exists, false otherwise.
       # Generally an object that isn't new will exist unless it has
       # been deleted.  Uses a database query to check for existence,
       # unless the model object is new, in which case this is always
       # false.
       #
-      #   Artist[1].exists? # SELECT 1 FROM artists WHERE (id = 1)
+      #   Artist[1].exists? # SELECT 1 AS one FROM artists WHERE (id = 1)
       #   # => true
       #   Artist.new.exists?
       #   # => false
       def exists?
-        new? ? false : !this.get(SQL::AliasedExpression.new(1, :one)).nil?
+        new? ? false : !this.get(EXISTS_SELECT_).nil?
       end
 
       # Ignore the model's setter method cache when this instances extends a module, as the

spec/model/record_spec.rb

@@ -1425,7 +1425,7 @@ def set_column_value(c, v)
     DB.sqls.must_equal ['SELECT 1 AS one FROM items WHERE (id = 1) LIMIT 1']
   end
 
-  it "should return false when #this.count == 0" do
+  it "should return false when #this.empty?" do
     @model.load(:id=>2).exists?.must_equal false
     DB.sqls.must_equal ['SELECT 1 AS one FROM items WHERE (id = 2) LIMIT 1']
   end