Batch etsource cache reads with fetch_multi to reduce N+1 queries

MeritOrder collapses 6 individual cache fetches into one fetch_multi call.
Molecules does the same for its 2 keys. Both use Thread.current to ensure
fetch_multi runs at most once per request.

Closes #1747

Author: aaccensi

app/models/etsource/merit_order.rb

@@ -1,5 +1,7 @@
 module Etsource
   class MeritOrder
+    CACHE_ATTRIBUTES = %i[merit_order hydrogen heat_network_lt heat_network_mt heat_network_ht agriculture_heat].freeze
+
     def initialize(etsource = Etsource::Base.instance)
       @etsource = etsource
     end
@@ -84,21 +86,34 @@ def import_agriculture_heat
     #
     # Returns a hash.
     def import(attribute)
-      Rails.cache.fetch("#{attribute}_hash") do
-        mo_nodes = Atlas::EnergyNode.all.select(&attribute).sort_by(&:key)
-        mo_data  = {}
+      warm_cache!
+      Thread.current[:merit_order_cache][attribute]
+    end
+
+    # Batches all MeritOrder cache reads into one SQL query per request, so
+    # subsequent fetch calls hit the LocalStore instead of the database.
+    def warm_cache!
+      cache_attr_by_key = CACHE_ATTRIBUTES.index_by { |a| "#{a}_hash" }
 
-        mo_nodes.each do |node|
-          config = node.public_send(attribute)
-          type   = config.type.to_s
-          group  = config.group&.to_s
+      Thread.current[:merit_order_cache] ||= Rails.cache.fetch_multi(*cache_attr_by_key.keys) do |cache_key|
+        compute(cache_attr_by_key[cache_key])
+      end.transform_keys { |k| cache_attr_by_key[k] }
+    end
+
+    def compute(attribute)
+      mo_nodes = Atlas::EnergyNode.all.select(&attribute).sort_by(&:key)
+      mo_data  = {}
 
-          mo_data[type] ||= {}
-          mo_data[type][node.key.to_s] = group
-        end
+      mo_nodes.each do |node|
+        config = node.public_send(attribute)
+        type   = config.type.to_s
+        group  = config.group&.to_s
 
-        mo_data
+        mo_data[type] ||= {}
+        mo_data[type][node.key.to_s] = group
       end
+
+      mo_data
     end
   end
 end

app/models/etsource/molecules.rb

@@ -3,6 +3,8 @@
 module Etsource
   # Loads data relating to the calculation of molecule flows based on the energy graph.
   module Molecules
+    CACHE_KEYS = %w[molecules.from_energy_keys molecules.from_molecules_keys].freeze
+
     module_function
 
     # Internal: Computes the list of molecule graph nodes which have a molecule_conversion
@@ -12,6 +14,7 @@ module Molecules
     #
     # Returns an Array of Symbols.
     def from_energy_keys
+      warm_cache!
       Rails.cache.fetch('molecules.from_energy_keys') do
         Atlas::MoleculeNode.all.select(&:from_energy).map(&:key).sort
       end
@@ -24,9 +27,21 @@ def from_energy_keys
     #
     # Returns an Array of Symbols.
     def from_molecules_keys
+      warm_cache!
       Rails.cache.fetch('molecules.from_molecules_keys') do
         Atlas::EnergyNode.all.select(&:from_molecules).map(&:key).sort
       end
     end
+
+    # Batches all Molecules cache reads into one SQL query per request, so
+    # subsequent fetch calls hit the LocalStore instead of the database.
+    def warm_cache!
+      Thread.current[:molecules_cache] ||= Rails.cache.fetch_multi(*CACHE_KEYS) do |key|
+        case key
+        when 'molecules.from_energy_keys'    then Atlas::MoleculeNode.all.select(&:from_energy).map(&:key).sort
+        when 'molecules.from_molecules_keys' then Atlas::EnergyNode.all.select(&:from_molecules).map(&:key).sort
+        end
+      end
+    end
   end
 end