Temporary transactions page performance fix (#2372)

* Temporary transactions page performance fix

* Fix Cursor bugs

* More bugbot bug fixes

Author: zachgoll

app/controllers/transactions_controller.rb

@@ -3,6 +3,8 @@ class TransactionsController < ApplicationController
 
   before_action :store_params!, only: :index
 
+  require "digest/md5"
+
   def new
     super
     @income_categories = Current.family.categories.incomes.alphabetically
@@ -15,35 +17,91 @@ def index
 
     set_focused_record(transactions_query, params[:focused_record_id], default_per_page: 50)
 
-    @pagy, @transactions = pagy(
-      transactions_query.includes(
-        { entry: :account },
-        :category, :merchant, :tags,
-        transfer_as_outflow: { inflow_transaction: { entry: :account } },
-        transfer_as_inflow: { outflow_transaction: { entry: :account } }
-      ).reverse_chronological,
-      limit: params[:per_page].presence || default_params[:per_page],
-      params: ->(params) { params.except(:focused_record_id) }
+    # ------------------------------------------------------------------
+    # Cache the expensive includes & pagination block so the DB work only
+    # runs when either the query params change *or* any entry has been
+    # updated for the current family.
+    # ------------------------------------------------------------------
+
+    latest_update_ts = Current.family.entries.maximum(:updated_at)&.utc&.to_i || 0
+
+    items_per_page = (params[:per_page].presence || default_params[:per_page]).to_i
+    items_per_page = 1 if items_per_page <= 0
+
+    current_page   = (params[:page].presence || default_params[:page]).to_i
+    current_page   = 1 if current_page <= 0
+
+    # Build a compact cache digest: sanitized filters + page info + a
+    # token that changes on updates *or* deletions.
+    entries_changed_token = [ latest_update_ts, Current.family.entries.count ].join(":")
+
+    digest_source = {
+      q:    @q,                 # processed & sanitised search params
+      page: current_page,       # requested page number
+      per:  items_per_page,     # page size
+      tok:  entries_changed_token
+    }.to_json
+
+    cache_key = Current.family.build_cache_key(
+      "transactions_idx_#{Digest::MD5.hexdigest(digest_source)}"
     )
 
-    # -------------------------------------------------------------------
-    # Cache totals
-    # -------------------------------------------------------------------
-    # Totals calculation is expensive (heavy SQL with grouping). We cache the
-    # result keyed by:
-    #   • Family id
-    #   • The family-level cache key that already embeds entries.maximum(:updated_at)
-    #   • A digest of the current search params so each distinct filter set gets
-    #     its own cache entry.
-    # When any entry is created/updated/deleted, the family cache key changes,
-    # automatically invalidating all related totals.
-
-    params_digest = Digest::MD5.hexdigest(@q.to_json)
-    cache_key     = Current.family.build_cache_key("transactions_totals_#{params_digest}")
-
-    @totals = Rails.cache.fetch(cache_key) do
-      Current.family.income_statement.totals(transactions_scope: transactions_query)
+    cache_data = Rails.cache.fetch(cache_key, expires_in: 30.minutes) do
+      current_page_i = current_page
+
+      # Initial query
+      offset = (current_page_i - 1) * items_per_page
+      ids = transactions_query
+              .reverse_chronological
+              .limit(items_per_page)
+              .offset(offset)
+              .pluck(:id)
+
+      total_count = transactions_query.count
+
+      if ids.empty? && total_count.positive? && current_page_i > 1
+        current_page_i = (total_count.to_f / items_per_page).ceil
+        offset = (current_page_i - 1) * items_per_page
+
+        ids = transactions_query
+                .reverse_chronological
+                .limit(items_per_page)
+                .offset(offset)
+                .pluck(:id)
+      end
+
+      { ids: ids, total_count: total_count, current_page: current_page_i }
     end
+
+    ids         = cache_data[:ids]
+    total_count = cache_data[:total_count]
+    current_page = cache_data[:current_page]
+
+    # Build Pagy object (this part is cheap – done *after* potential
+    # page fallback so the pagination UI reflects the adjusted page
+    # number).
+    @pagy = Pagy.new(
+      count:  total_count,
+      page:   current_page,
+      items:  items_per_page,
+      params: ->(p) { p.except(:focused_record_id) }
+    )
+
+    # Fetch the transactions in the cached order
+    @transactions = Current.family.transactions
+                     .active
+                     .where(id: ids)
+                     .includes(
+                       { entry: :account },
+                       :category, :merchant, :tags,
+                       transfer_as_outflow: { inflow_transaction: { entry: :account } },
+                       transfer_as_inflow: { outflow_transaction: { entry: :account } }
+                     )
+
+    # Preserve the order defined by `ids`
+    @transactions = ids.map { |id| @transactions.detect { |t| t.id == id } }.compact
+
+    @totals = Current.family.income_statement.totals(transactions_scope: transactions_query)
   end
 
   def clear_filter