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

app/models/etsource/merit_order.rb

@@ -1,5 +1,18 @@
 module Etsource
   class MeritOrder
+    class << self
+      attr_accessor :cache
+    end
+
+    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 +97,39 @@ 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  = {}
+      merit_order_cache[attribute]
+    end
 
-        mo_nodes.each do |node|
-          config = node.public_send(attribute)
-          type   = config.type.to_s
-          group  = config.group&.to_s
+    def self.reset_cache!
+      self.cache = nil
+    end
 
-          mo_data[type] ||= {}
-          mo_data[type][node.key.to_s] = group
-        end
+    # Batches all MeritOrder cache reads into one SQL query per request, so
+    # subsequent fetch calls are served from memory instead of the database.
+    def merit_order_cache
+      self.class.cache ||= begin
+        cache_attr_by_key = CACHE_ATTRIBUTES.index_by { |a| "#{a}_hash" }
 
-        mo_data
+        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
     end
+
+    def compute(attribute)
+      mo_nodes = Atlas::EnergyNode.all.select(&attribute).sort_by(&:key)
+      mo_data  = {}
+
+      mo_nodes.each do |node|
+        config = node.public_send(attribute)
+        type   = config.type.to_s
+        group  = config.group&.to_s
+
+        mo_data[type] ||= {}
+        mo_data[type][node.key.to_s] = group
+      end
+
+      mo_data
+    end
   end
 end

app/models/etsource/molecules.rb

@@ -3,6 +3,11 @@
 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
@@ -12,9 +17,7 @@ module Molecules
     #
     # Returns an Array of Symbols.
     def from_energy_keys
-      Rails.cache.fetch('molecules.from_energy_keys') do
-        Atlas::MoleculeNode.all.select(&:from_energy).map(&:key).sort
-      end
+      molecule_cache['molecules.from_energy_keys']
     end
 
     # Internal: Computes the list of molecule graph nodes which have a molecule_conversion
@@ -24,8 +27,21 @@ def from_energy_keys
     #
     # Returns an Array of Symbols.
     def from_molecules_keys
-      Rails.cache.fetch('molecules.from_molecules_keys') do
-        Atlas::EnergyNode.all.select(&:from_molecules).map(&:key).sort
+      molecule_cache['molecules.from_molecules_keys']
+    end
+
+    def reset_cache!
+      @molecule_cache = nil
+    end
+
+    # Batches all Molecules cache reads into one SQL query per request, so
+    # subsequent fetch calls are served from memory instead of the database.
+    def molecule_cache
+      @molecule_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

app/models/nasty_cache.rb

@@ -145,6 +145,11 @@ def expire_local!
     Merit::Curve.reader = Merit::Curve.reader.class.new
 
     @cache_store = {}
+
+    # Reset process-level memoization so the next request re-fetches from
+    # Rails.cache, which has just been cleared by expire_cache!
+    Etsource::MeritOrder.reset_cache!
+    Etsource::Molecules.reset_cache!
   end
 
   def expire_atlas!(options)