Use cached fields for item searches, instead of big joins

This is the second part of the previous change `efda6d74`, in which we
switch out the item search query conditions!

This was a two-parter to ease deployment: first deploy the change with
the migration, then *run* the migration (because it's an unusually slow
one), then deploy this change that actually uses it.

Another way to approach this would've been to deploy it all in one
commit, but not set it as `current` until we had run the migration.
That would have been a reasonable approach too!

app/models/item.rb

@@ -11,10 +11,10 @@ class Item < ApplicationRecord
   SwfAssetType = 'object'
 
   has_many :closet_hangers
-  has_one :contribution, :as => :contributed, :inverse_of => :contributed
+  has_one :contribution, as: :contributed, inverse_of: :contributed
   has_one :nc_mall_record
-  has_many :parent_swf_asset_relationships, :as => :parent
-  has_many :swf_assets, :through => :parent_swf_asset_relationships
+  has_many :parent_swf_asset_relationships, as: :parent
+  has_many :swf_assets, through: :parent_swf_asset_relationships
   belongs_to :dyeworks_base_item, class_name: "Item",
     default: -> { inferred_dyeworks_base_item }, optional: true
   has_many :dyeworks_variants, class_name: "Item",
@@ -61,38 +61,18 @@ class Item < ApplicationRecord
       '%' + sanitize_sql_like(PAINTBRUSH_SET_DESCRIPTION) + '%')
   }
   scope :occupies, ->(zone_label) {
-    zone_ids = Zone.matching_label(zone_label).map(&:id)
-
-    # NOTE: In searches, this query performs much better using a subquery
-    # instead of joins! This is because, in the joins case, filtering by an
-    # `swf_assets` field but sorting by an `items` field causes the query
-    # planner to only be able to use an index for *one* of them. In this case,
-    # MySQL can use the `swf_assets`.`zone_id` index to get the item IDs for
-    # the subquery, then use the `items`.`name` index to sort them.
-    i = arel_table
-    psa = ParentSwfAssetRelationship.arel_table
-    sa = SwfAsset.arel_table
-    where(
-      ParentSwfAssetRelationship.joins(:swf_asset).
-        where(sa[:zone_id].in(zone_ids)).
-        where(psa[:parent_type].eq("Item")).
-        where(psa[:parent_id].eq(i[:id])).
-        arel.exists
-    )
+    Zone.matching_label(zone_label).
+      map { |z| occupies_zone_id(z.id) }.reduce(none, &:or)
   }
   scope :not_occupies, ->(zone_label) {
-    zone_ids = Zone.matching_label(zone_label).map(&:id)
-    i = Item.arel_table
-    sa = SwfAsset.arel_table
-    # Querying for "has NO swf_assets matching these zone IDs" is trickier than
-    # the positive case! To do it, we GROUP_CONCAT the zone_ids together for
-    # each item, then use FIND_IN_SET to search the result for each zone ID,
-    # and assert that it must not find a match. (This is uhh, not exactly fast,
-    # so it helps to have other tighter conditions applied first!)
-    # TODO: I feel like this could also be solved with a LEFT JOIN, idk if that
-    # performs any better? In Rails 5+ `left_outer_joins` is built in so!
-    condition = zone_ids.map { 'FIND_IN_SET(?, GROUP_CONCAT(zone_id)) = 0' }.join(' AND ')
-    joins(:swf_assets).group(i[:id]).having(condition, *zone_ids).distinct
+    Zone.matching_label(zone_label).
+      map { |z| not_occupies_zone_id(z.id) }.reduce(all, &:and)
+  }
+  scope :occupies_zone_id, ->(zone_id) {
+    where("FIND_IN_SET(?, cached_occupied_zone_ids) > 0", zone_id)
+  }
+  scope :not_occupies_zone_id, ->(zone_id) {
+    where.not("FIND_IN_SET(?, cached_occupied_zone_ids) > 0", zone_id)
   }
   scope :restricts, ->(zone_label) {
     zone_ids = Zone.matching_label(zone_label).map(&:id)
@@ -105,31 +85,12 @@ class Item < ApplicationRecord
     where("NOT (#{condition})", *zone_ids)
   }
   scope :fits, ->(body_id) {
-    joins(:swf_assets).where(swf_assets: {body_id: [body_id, 0]}).distinct
+    where("FIND_IN_SET(?, cached_compatible_body_ids) > 0", body_id).
+      or(where("FIND_IN_SET('0', cached_compatible_body_ids) > 0"))
   }
   scope :not_fits, ->(body_id) {
-    i = Item.arel_table
-    sa = SwfAsset.arel_table
-    # Querying for "has NO swf_assets matching these body IDs" is trickier than
-    # the positive case! To do it, we GROUP_CONCAT the body_ids together for
-    # each item, then use FIND_IN_SET to search the result for the body ID,
-    # and assert that it must not find a match. (This is uhh, not exactly fast,
-    # so it helps to have other tighter conditions applied first!)
-    #
-    # TODO: I feel like this could also be solved with a LEFT JOIN, idk if that
-    # performs any better? In Rails 5+ `left_outer_joins` is built in so!
-    #
-    # NOTE: The `fits` and `not_fits` counts don't perfectly add up to the
-    # total number of items, 5 items aren't accounted for? I'm not going to
-    # bother looking into this, but one thing I notice is items with no assets
-    # somehow would not match either scope in this impl (but LEFT JOIN would!)
-    joins(:swf_assets).group(i[:id]).
-      having(
-        "FIND_IN_SET(?, GROUP_CONCAT(body_id)) = 0 AND " +
-        "FIND_IN_SET(0, GROUP_CONCAT(body_id)) = 0",
-        body_id
-      ).
-      distinct
+    where.not("FIND_IN_SET(?, cached_compatible_body_ids) > 0", body_id).
+      and(where.not("FIND_IN_SET('0', cached_compatible_body_ids) > 0"))
   }
 
   def nc_trade_value