Rails joins with limit on association

Question

I´m trying to do a query for all Stations, and join Measures But I only want the most recent measure (ordered by created_at DESC) since a station has thousands of measures. I have tried ``` Station.joins(:measures).limit(1) ``` but that just limits the Stations. Additional info: Station has many Measures Measure belongs to Station I have read through the Active Records docs and there is only information on using a where condition on associations. The app only targets Postgres, SQL is accepted. Edit: added except from schema.rb: ``` create_table "measures", force: true do |t| t.integer "station_id" t.float "speed" t.float "direction" t.float "max_wind_speed" t.float "min_wind_speed" t.float "temperature" t.datetime "created_at" t.datetime "updated_at" t.float "speed_calibration" end add_index "observations", ["created_at"], name: "index_observations_on_created_at", using: :btree add_index "observations", ["station_id"], name: "index_observations_on_station_id", using: :btree create_tab…

Accepted Answer

I believe in Rails 4 you can apply a scope on the association:

```
class Stations
  has_many :measures, -> { order('created_at DESC').limit(1)  }
end
```

Then:

```
2.0.0-p353 :008 > Station.first.measures
  Station Load (0.1ms)  SELECT "stations".* FROM "stations" ORDER BY "stations"."id" ASC LIMIT 1
  Measure Load (0.1ms)  SELECT "measures".* FROM "measures" WHERE "measures"."station_id" = ? ORDER BY created_at DESC LIMIT 1  [["station_id", 1]]
```

Edit: Actually if you need only the most recent one you can use `has_one`. It will work both for Rails 4 and Rails 3, with slightly modified syntax:

```
class Stations
  has_one :recent_measure, -> { order('created_at DESC')  }, class_name: 'Measure' # Rails 4
  has_one :recent_measure, order: 'created_at DESC', class_name: 'Measure' # Rails 3
end
```

Rails joins with limit on association

Solution

Problem