From a410d15dcffbb6f890b080351c79b0bb7d771a28 Mon Sep 17 00:00:00 2001
From: suttondemlong <69323179+suttondemlong@users.noreply.github.com>
Date: Fri, 21 Aug 2026 15:47:11 +0000
Subject: [PATCH] Filter checkins by date in Mongo instead of in Ruby

CheckinsController#index called .select with a block on the criteria,
which is Enumerable#select rather than Mongoid::Criteria#select. That
loaded every checkin the user had ever created, along with the eager
loaded associations for all of them, and then threw away everything
outside the requested day. Cost grew with account age on the endpoint
the clients hit to render a single day.

Use the existing by_date scope over the day's bounds so the query is
served by the existing index(date: 1, encrypted_user_id: 1), and
materialise with to_a so the criteria is not enumerated once per
serializer pass.

Measured with a user holding 60 days of checkins: before, one unfiltered
find returning 60 documents; after, one filtered find returning 1, with
the same total number of Mongo commands.

Fixes #874

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 .../controllers/api/v1/checkins_controller.rb |  7 ++--
 .../api/v1/checkins_controller_spec.rb        |  9 +++++
 backend/spec/support/mongo_commands.rb        | 39 +++++++++++++++++++
 3 files changed, 52 insertions(+), 3 deletions(-)
 create mode 100644 backend/spec/support/mongo_commands.rb

diff --git a/backend/app/controllers/api/v1/checkins_controller.rb b/backend/app/controllers/api/v1/checkins_controller.rb
index 1710c8fc7..901cbc415 100644
--- a/backend/app/controllers/api/v1/checkins_controller.rb
+++ b/backend/app/controllers/api/v1/checkins_controller.rb
@@ -7,9 +7,10 @@ def index
         if date.blank? && params.require(:page)
           render json: current_user.checkins.where(:note.nin => [nil, ""]).order_by(date: :desc).page(params[:page]).per(10)
         else
-          render json: current_user.checkins.includes([:harvey_bradshaw_index, :promotion_rate, :conditions, :symptoms, :treatments]).select { |x|
-            x.date.to_date == Date.parse(date)
-          }
+          day = Date.parse(date)
+          checkins = current_user.checkins.by_date(day.beginning_of_day, day.end_of_day)
+          # to_a so the criteria is loaded and eager loaded once, not once per serializer pass
+          render json: checkins.includes([:harvey_bradshaw_index, :promotion_rate, :conditions, :symptoms, :treatments]).to_a
         end
       end
 
diff --git a/backend/spec/controllers/api/v1/checkins_controller_spec.rb b/backend/spec/controllers/api/v1/checkins_controller_spec.rb
index 03a846906..7f957b776 100644
--- a/backend/spec/controllers/api/v1/checkins_controller_spec.rb
+++ b/backend/spec/controllers/api/v1/checkins_controller_spec.rb
@@ -22,6 +22,15 @@
         returned_checkin = response_body[:checkins][0]
         expect(Date.parse(returned_checkin[:date])).to eq Date.parse(date)
       end
+
+      it "filters by date in the database instead of loading every checkin" do
+        commands = capture_mongo_commands { get :index, params: {date: date} }
+
+        checkin_finds = commands.select { |command| command.command["find"] == "checkins" }
+
+        expect(checkin_finds.size).to eq 1
+        expect(checkin_finds.first.command["filter"]["date"].keys).to match_array ["$gte", "$lte"]
+      end
     end
     context "when checkin doesn't exist for the passed date" do
       it "returns no results" do
diff --git a/backend/spec/support/mongo_commands.rb b/backend/spec/support/mongo_commands.rb
new file mode 100644
index 000000000..6f3c66659
--- /dev/null
+++ b/backend/spec/support/mongo_commands.rb
@@ -0,0 +1,39 @@
+# Captures the commands the Mongo driver sends while a block runs, so specs can
+# assert that filtering happens in the database rather than in Ruby.
+module MongoCommands
+  class Subscriber
+    attr_reader :commands
+
+    def initialize
+      @commands = []
+    end
+
+    def started(event)
+      @commands << event
+    end
+
+    def succeeded(event)
+    end
+
+    def failed(event)
+    end
+  end
+
+  def capture_mongo_commands
+    subscriber = Subscriber.new
+    client = Mongoid.default_client
+    client.subscribe(Mongo::Monitoring::COMMAND, subscriber)
+
+    begin
+      yield
+    ensure
+      client.unsubscribe(Mongo::Monitoring::COMMAND, subscriber)
+    end
+
+    subscriber.commands
+  end
+end
+
+RSpec.configure do |config|
+  config.include MongoCommands, type: :controller
+end