Question

I’m using Postgres with Rails. There’s a query with a subselect which returns a boolean, but Postgres always returns a String like 't' or 'f'. But in the generated JSON I need a real boolean.

This is my query:

SELECT
  *,
  EXISTS (
    SELECT TRUE
    FROM measurement
    JOIN experiment ON measurement.experiment_id = experiment.id
    JOIN location ON experiment.location_id = location.id
    WHERE location.map_id = map.id
    LIMIT 1
  ) AS measurements_exist
FROM "map"

It doesn’t matter whether I use THEN true or THEN 1 or THEN 'true', I will always get a string. So my JSON response will always look like that:

[
  {"id":8, ..., "measurements_exist":"f"},
  {"id":9, ..., "measurements_exist":"t"}
]

But it should(!) look like that:

[
  {"id":8, ..., "measurements_exist":false},
  {"id":9, ..., "measurements_exist":true}
]

Is there any way to get this working right?

Thank you!

---

THE SOLUTION:

Just give the corresponding model (here: Map) an attribute accessor, which uses value_as_boolean to convert the value. So every time the controller tries to access the value, it uses the attribute accessor method automatically.

The controller code:

class MapsController < ApplicationController
  def index
    select = ["*"]
    select.push(measurements_exist) # This will just insert the string returned by the 'measurements_exist' method
    maps = Map.select(select) # Results in 'SELECT *, EXISTS (...) AS measurements_exist FROM "map"'
    render json: maps
  end

  private

  def measurements_exist
    "EXISTS (
      SELECT TRUE
      FROM measurement
      JOIN experiment ON measurement.experiment_id = experiment.id
      JOIN location ON experiment.location_id = location.id
      WHERE location.map_id = map.id
      LIMIT 1
    ) AS measurements_exist"
  end
end

The model code:

class Map < ActiveRecord::Base
  def measurements_exist
    ActiveRecord::ConnectionAdapters::Column.value_to_boolean(self[:measurements_exist])
  end
end

Resulting JSON:

[
  {"id":7, ..., "measurements_exist":false},
  {"id":6, ..., "measurements_exist":true}
]

Answer 1

ActiveRecord has a method called ActiveRecord::ConnectionAdapters::Column.value_to_boolean it uses internally to convert any true-like value to a Ruby true value.

You can use it in your code.