Set default value for Postgres JSON column in Rails < 4

Question

So I'm starting to use the Postgres JSON datatype, now that there's a lot of fun stuff you can do with it. In one of my Rails apps which is not yet Rails 4 (where support for Postgres JSON has been added) I added a JSON column like this:

create_table :foo do |t|
  t.column :bar, :json
end

but I can't figure out how to set a default value for the column. I tried all variations like {}, '{}', '{}'::json, '[]'::json etc. but I either get an error when the migration runs or it simply doesn't work, meaning the migration runs but, when I create a new Foo, bar is nil.

Mendive

Have you tried setting the default value by hand inside the model? AR generally butchers or ignores defaults that it doesn't understand. – Ambassador

Yeah, right now I use a after_initialize callback but I generally don't like those... – Mendive

try "", I believe that is what is used for hstore to denote an empty hash – Bukavu

-- add_column(:foo, :bar, :json, {:default=>""}) rake aborted! An error has occurred, this and all later migrations canceled: PG::InvalidTextRepresentation: ERROR: invalid input syntax for type json DETAIL: The input string ended unexpectedly. CONTEXT: JSON data, line 1: : ALTER TABLE "foo" ADD COLUMN "bar" json DEFAULT '' – Mendive

Can you try setting default to "null"? – Umeh

Well, it's null by default... by default. I don't want it to be null, I want it to be {}. – Mendive

Answer 1

Although a bit late, this worked for me (requires Postgres >= 9.3):

create_table :foo do |t|
  t.column :bar, :json
end

execute "ALTER TABLE foo ALTER COLUMN bar SET DEFAULT '[]'::JSON"

EDIT: this answer used to advocate for to_json('[]'::text) instead of '[]'::JSON - thanks to @Offirmo for the hint.

The problem with the old method was that it didn't actually define an array or an object as the default value as one would expect, but a scalar (string) that looked like one. Why does that matter?

Postgres allows three kinds of values to be inserted into JSON columns:

1. Objects

   INSERT INTO foo (bar) VALUE('{}')

2. Arrays

   INSERT INTO foo (bar) VALUE('[]')

3. Scalars

   INSERT INTO foo (bar) VALUE('"string"')

The problem is that if you mix these three kinds in the same column, you lose the ability to use the JSON operators. If you set a default of '[]' using the previously advocated method and queried for an array element, encountering a single row with a scalar default value would abort the whole query with an error:

=# SELECT * FROM foo WHERE bar->>1 = 'baz';
ERROR:  cannot extract element from a scalar

Pisces

I get the following error: PG::UndefinedFunction: ERROR: function to_json(text) does not exist HINT: No function matches the given name and argument types. You might need to add explicit type casts. – Mendive

@ManuelMeurer I believe you need a recent version of Postgres for that; I am on 9.3.2. – Pisces

Yeah it was added in 9.3. Could you mention that in your answer? Then I can accept it. – Mendive

One more tiny thing, can you change the > to >=? Cheers! – Mendive

This syntax also works : SET DEFAULT '{}'::JSON; I find it cleaner. – Reggie

@Reggie thank you! Your answer is the one that actually works - mine was subtly broken. – Pisces

@ManuelMeurer if you are still working with the JSON datatype, you might be interested in the updated answer. – Pisces

Answer 2

Code below works for PostgreSQL 9.3.4 and Rails 3.2.17

class YourModel < ActiveRecord::Base
...
  serialize :your_column, JSON
  before_create do
    self.your_column ||= {}
  end
...
end

migration code

add_column :your_table, :your_column, :json
execute "ALTER TABLE your_table ALTER COLUMN your_column SET DEFAULT '{}'"
execute "UPDATE your_table SET your_column = '{}';"

application.rb

config.active_record.schema_format = :sql

Oneill

Answer 3

You can basically just do something like this

create_table :foo do |t|
  t.column :bar, :json, default: []
end

State the default with default: [] or default: {} or any other thing you feel comfortable with.

Update: Note that this is for Rails 4+

Emotionalism

As mentioned in my question 6.5 years ago, the app was still on Rails 3, so this won't (didn't) work. Could you improve your answer by mentioning from which Rails version on your suggestion works? – Mendive