ordering through an has_many relationship with a condition in rails 3 / active records / postgresql -

i have 2 objects, items , taxonomy nodes.

i have selection of "items" want order based on alphabetical order of taxonomy_nodes attached to, 1 particular "top parent", id provided (as in "i want sort alphabetically using parent"). if understood nested sets properly, can find out if taxonomy_node belongs parent when left , right values "within" left , right values of top parent: instance top node "vegetable" of values left:1 , right: 10 has "potato" of values left: 2 , right: 8 child.

an item can have several taxonomy_nodes, , using awesome_nested_set gem taxonomy_nodes behave nested sets hierarchy. thus, can have various main "taxonomies" against items tagged, identified "top node".

the way, moment, identify if taxonomy_node belongs particular "top node" use lft , rght values of taxonomy_node, , hit stumbling block when trying allow users sort list of items 1 of main taxonomies.

i imagine way achieve allowed use "conditions" applied in ordering itself, cannot how possible!

ordering more simple field, "item_template", achieved:

 @items.joins(:item_template).includes(:item_template}.order("template_name #{sorting_direction}") 

here taxonomy table:

table name: taxonomies  id              :integer          not null, primary key name            :string(255) parent_id       :integer lft             :integer rgt             :integer created_at      :datetime         not null updated_at      :datetime         not null depth           :integer 

the taxonomies related items through itemtaxonomy object. not overly familiar postgresql, , wondering if maybe write function try , achieve result?

hope clear!

^{new version after update.}
can join items taxonomies , taxonomies must lie within range of "top parent" - can implemented join. expression in where clause well.

assuming nested set model per updated description, lft & right within range of superior node.

also assuming:

• items without link qualifying taxonomy dropped result
• items multiple links qualifying taxonomies listed multiple times.

select t.name, i.*   item join   taxonomy_node tn on tn.item_id = i.id  -- assuming, no specification join   taxonomies t on t.id = tn.taxonomy_id  -- assuming ... join   taxonomies parent on parent.lft  <= t.lft                            , parent.rght >= t.rght  parent.id = ?    -- id of "top parent" here order  t.name