Drastically reduce queries for item page preview

Oh right okay, I made a sloppy perf hack long ago, and now let's
actually clean it up!

app/models/item.rb

@@ -555,10 +555,10 @@ class Item < ApplicationRecord
     # Otherwise, create an appearance for each real (nonzero) body ID. We don't
     # generally expect body_id = 0 and body_id != 0 to mix, but if they do,
     # uhh, let's merge the body_id = 0 ones in?
+    species_by_body_id = Species.with_body_ids(swf_assets_by_body_id.keys)
     swf_assets_by_body_id.map do |body_id, body_specific_assets|
       swf_assets_for_body = body_specific_assets + swf_assets_for_all_bodies
-      species = Species.with_body_id(body_id).first!
+      body = Appearance::Body.new(body_id, species_by_body_id[body_id])
-      body = Appearance::Body.new(body_id, species)
       Appearance.new(self, body, swf_assets_for_body)
     end
   end

app/models/species.rb

@@ -3,11 +3,6 @@ class Species < ApplicationRecord
   has_many :alt_styles
   
   scope :alphabetical, -> { order(:name) }
-
-  scope :with_body_id, -> body_id {
-    pt = PetType.arel_table
-    joins(:pet_types).where(pt[:body_id].eq(body_id)).limit(1)
-  }
   
   def as_json(options={})
     super({only: [:id, :name], methods: [:human_name]}.merge(options))
@@ -20,4 +15,15 @@ class Species < ApplicationRecord
       I18n.translate('species.default_human_name')
     end
   end
+
+  # Given a list of body IDs, return a hash from body ID to Species.
+  # (We assume that each body ID belongs to just one species; if not, which
+  # species we return for that body ID is undefined.)
+  def self.with_body_ids(body_ids)
+    species_ids_by_body_id = PetType.where(body_id: body_ids).distinct.
+      pluck(:body_id, :species_id).to_h
+    species_by_id = Species.where(id: species_ids_by_body_id.values).
+      to_h { |s| [s.id, s] }
+    species_ids_by_body_id.transform_values { |id| species_by_id[id] }
+  end
 end