如何编写迁移以将 JSON 字段转换为 Postgres 数组以在 rails 中查询？

Question

There is an old table with column type as JSON but only arrays are stored in this column. Even though I am storing array, I am not able to query this field using the ANY keyword (which will work on array type columns in Postgres like in this post )

Eg : let's say ['Apple', 'Orange', 'Banana'] is stored as Json in the fruits column, I want to query like Market.where(":name = ANY(fruits)", name: "Orange") and get all the markets with Oranges available.

Can anyone please help me to write a migration to change the existing column(type: Json) to array type?

Answer 1

One example assuming a json field:

\d json_test 
               Table "public.json_test"
  Column   |  Type   | Collation | Nullable | Default 
-----------+---------+-----------+----------+---------
 id        | integer |           |          | 
 fld_json  | json    |           |          | 
 fld_jsonb | jsonb   |           |          | 
 fruits    | json    |           |          | 

insert into json_test (id, fruits) values (1, '["Apple", "Orange", "Banana"] ');
insert into json_test (id, fruits) values (2, '["Pear", "Orange", "Banana"] ');
insert into json_test (id, fruits) values (3, '["Pear", "Apple", "Banana"] ');

WITH fruits AS 
(SELECT 
    id, json_array_elements_text(fruits) fruit 
 FROM json_test) 
SELECT 
   id 
FROM 
   fruits
WHERE 
    fruit = 'Orange';
 id 
----
  1
  2

UPDATE Method to convert JSON array into Postgres array:

SELECT 
    array_agg(fruit) 
FROM 
    (SELECT 
        id, json_array_elements_text(fruits)AS fruit 
    FROM 
    json_test) AS elements 
GROUP BY 
    id;

 array_agg       
-----------------------
 {Pear,Apple,Banana}
 {Pear,Orange,Banana}
 {Apple,Orange,Banana}

This assumes the JSON array has homogeneous elements as that is a requirement for Postgres arrays.

A simpler method of finding rows that have 'Orange' in the json field:

SELECT 
    id, fruits 
FROM 
    json_test 
WHERE 
    fruits::jsonb ? 'Orange';

 id |             fruits             
----+--------------------------------
  1 | ["Apple", "Orange", "Banana"] 
  2 | ["Pear", "Orange", "Banana"] 

Answer 2

class AddArrayFruitsToMarkets < ActiveRecord::Migration[6.0]
  def up
    rename_column :markets, :fruits, :old_fruits
    add_column :markets, :fruits, :string, array: true
    Market.update_all('fruits = json_array_elements(old_fruits)')
  end
end

class RemoveJsonFruitsFromMarkets < ActiveRecord::Migration[6.0]
  def up
    remove_column :markets, :old_fruits
  end
end

But really if you're going to do something why not create tables instead as you're not really improving anything?

class Fruit < ApplicationRecord
  validates :name, presence: true
  has_many :market_fruits
  has_many :markets, through: :market_fruits
end

class MarketFruit < ApplicationRecord
  belongs_to :market
  belongs_to :fruit
end

class Market < ApplicationRecord
  has_many :market_fruits
  has_many :fruits, through: :market_fruits

  def self.with_fruit(name)
    joins(:fruits)
      .where(fruits: { name: name })
  end

  def self.with_fruits(*names)
    left_joins(:fruits)
      .group(:id)
      .where(fruits: { name: names })
      .having('COUNT(fruits.*) >= ?', names.length) 
  end
end