2 files changed, 14 insertions, 3 deletions
diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 9a2900843e..ff0fbe932b 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -370,7 +370,7 @@ module ActiveRecord
           if record.new_record?
             include_in_memory?(record)
           else
-            loaded? ? target.include?(record) : scope.exists?(record)
+            loaded? ? target.include?(record) : scope.exists?(record.id)
           end
         else
           false
diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index 5ebfc318aa..7192aecee1 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -1,3 +1,5 @@
+require 'active_support/deprecation'
+
 module ActiveRecord
   module FinderMethods
     ONE_AS_ONE = '1 AS one'
@@ -280,7 +282,12 @@ module ActiveRecord
     #   Person.exists?(false)
     #   Person.exists?
     def exists?(conditions = :none)
-      conditions = conditions.id if Base === conditions
+      if Base === conditions
+        conditions = conditions.id
+        ActiveSupport::Deprecation.warn "You are passing an instance of ActiveRecord::Base to `exists?`." \
+          "Please pass the id of the object by calling `.id`"
+      end
+
       return false if !conditions
 
       relation = apply_join_dependency(self, construct_join_dependency)
@@ -412,7 +419,11 @@ module ActiveRecord
     end
 
     def find_one(id)
-      id = id.id if ActiveRecord::Base === id
+      if ActiveRecord::Base === id
+        id = id.id
+        ActiveSupport::Deprecation.warn "You are passing an instance of ActiveRecord::Base to `find`." \
+          "Please pass the id of the object by calling `.id`"
+      end
 
       column = columns_hash[primary_key]
       substitute = connection.substitute_at(column, bind_values.length)