SQL 语句将 jsonb hash 转换为 json 字符串

Question

I have a rails data migration (postgres db) where I have to use pure sql to convert the data due to some model restrictions. The data is stored as json as a string, but I need it to be a usable hash for other purposes.

My migration works to convert it to the hash. However, my down method ends up just deleting the data or leaving it as an empty {}. Btw to clear up any confusion, my column name is actually saved as data in table Games

Based on my up method, how would i properly reverse the migration using sql only?

class ConvertGamesDataToJson < ActiveRecord::Migration[6.0]
  def up
    statement = <<~SQL
      update games set data = regexp_replace(trim(both '"' from data::text), '\\\\"', '"', 'g')::jsonb;
    SQL

    ActiveRecord::Base.connection.execute(statement)
    # this part works!
  end

  def down
    statement = <<~SQL
      update games set data = to_json(data::text)::jsonb;
    SQL

    ActiveRecord::Base.connection.execute(statement)
  end
end

Here is how the it looks after properly converting it

data: {
  "id"=>"d092a-f2323",
  "recent"=>'yes',
  "note"=>"some text",
  "order"=>1
}

how it is before the migration and what it needs to rollback to:

data: 
  "{
    \"id\":\"d092a-f2323\",
    \"recent\":\"yes\",
    \"note\":\"some text\",
    \"order\":1,
  }"

Answer 1

If you're displaying a data structure in the rails console, those \" aren't really there. They're just formatting because the console has wrapped the string in " . For example...

[2] pry(main)> %{"up": "down"}
=> "\"up\": \"down\""

But if we print it...

[3] pry(main)> puts %{"up": "down"}
"up": "down"

Given that is a JSON string, you can simply change the type of the column to jsonb and be done with it.

-- up
alter table games alter column data type jsonb USING data::jsonb;

-- down
alter table games alter column data type text;

Postgres doesn't know how to automatically cast text to jsonb, so we need to tell it. using data::jsonb does a simple cast of the text to jsonb. It can cast jsonb to text just fine.

You can do this in a migration with change_column .

def up
  change_column :users, :data, :jsonb, using: 'data::jsonb'
end

def down
  change_column :users, :data, :text
end