While merging relations preserve context for joins

Fixes #3002. Also see #5494.

```
class Comment < ActiveRecord::Base
  belongs_to :post
end

class Author < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :author
  has_many :comments
end
```

`Comment.joins(:post).merge(Post.joins(:author).merge(Author.where(:name => "Joe Blogs"))).all` would
fail with `ActiveRecord::ConfigurationError: Association named 'author' was not found on Comment`.

It is failing because `all` is being called on relation which looks like this after all the merging:
`{:joins=>[:post, :author], :where=>[#<Arel::Nodes::Equality: ....}`. In this relation all the context that
`Post` was joined with `Author` is lost and hence the error that `author` was not found on `Comment`.

Ths solution is to build JoinAssociation when two relations with join information are being merged. And later
while building the arel use the  previously built `JoinAssociation` record in `JoinDependency#graft` to
build the right from clause.

Thanks to Jared Armstrong (https://github.com/armstrjare) for most of the work. I ported it to make it
compatible with new code base.

1 files changed, 30 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/relation/merger.rb b/activerecord/lib/active_record/relation/merger.rb
index eb23e92fb8..c7f3b88534 100644
--- a/activerecord/lib/active_record/relation/merger.rb
+++ b/activerecord/lib/active_record/relation/merger.rb
@@ -39,7 +39,7 @@ module ActiveRecord
     end
 
     class Merger # :nodoc:
-      attr_reader :relation, :values
+      attr_reader :relation, :values, :other
 
       def initialize(relation, other)
         if other.default_scoped? && other.klass != relation.klass
@@ -48,11 +48,12 @@ module ActiveRecord
 
         @relation = relation
         @values   = other.values
+        @other    = other
       end
 
       NORMAL_VALUES = Relation::SINGLE_VALUE_METHODS +
                       Relation::MULTI_VALUE_METHODS -
-                      [:where, :order, :bind, :reverse_order, :lock, :create_with, :reordering, :from] # :nodoc:
+                      [:joins, :where, :order, :bind, :reverse_order, :lock, :create_with, :reordering, :from] # :nodoc:
 
       def normal_values
         NORMAL_VALUES
@@ -66,12 +67,39 @@ module ActiveRecord
 
         merge_multi_values
         merge_single_values
+        merge_joins
 
         relation
       end
 
       private
 
+      def merge_joins
+        return if values[:joins].blank?
+
+        if other.klass == relation.klass
+          relation.joins! *values[:joins]
+        else
+          joins_dependency, rest = values[:joins].partition do |join|
+            case join
+            when Hash, Symbol, Array
+              true
+            else
+              false
+            end
+          end
+
+          join_dependency = ActiveRecord::Associations::JoinDependency.new(other.klass, 
+                                                                           joins_dependency,
+                                                                           [])
+          relation.joins! rest
+
+          join_dependency.join_associations.each do |association|
+            @relation = association.join_relation(relation)
+          end
+        end
+      end
+
       def merge_multi_values
         relation.where_values = merged_wheres
         relation.bind_values  = merged_binds