需要将布尔值从Postgres（== String）转换为Ruby布尔值 [英] Need to convert a Boolean from Postgres (== String) to a Ruby Boolean

问题描述

我正在将Postgres与Rails结合使用。有一个带有子选择的查询，该查询返回一个布尔值，但是Postgres总是返回一个字符串，例如't'或'f'。但是在生成的JSON中，我需要一个真正的布尔值。

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.

这是我的查询：

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"

我是否使用 THEN true 或都没关系然后1 或 true 时，我总是得到一个字符串。因此，我的JSON响应将始终如下所示：

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"}
]

但是它应该像这样：

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

有什么方法可以使此工作正常吗？

Is there any way to get this working right?

谢谢！

只需为相应的模型（此处为 Map ）提供一个属性访问器，该访问器便使用 value_as_boolean 来转换值。因此，每次控制器尝试访问该值时，它都会自动使用属性访问器方法。

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.

控制器代码：

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

模型代码：

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

结果JSON：

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

推荐答案

ActiveRecord的方法称为 ActiveRecord :: ConnectionAdapters :: Column.value_to_boolean ，它在内部将所有类似true的值转换为Ruby true 值。

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.

您可以在代码中使用它。

You can use it in your code.

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