问题:尝试使用 PostgreSQL 在 Rails 4 中创建 json 对象数组时出错

我正在尝试将一组 json 对象添加到我的一个模型中。只要我不包含默认值,运行迁移就可以正常运行,但是尝试存储任何值都会导致崩溃。当尝试使用空数组作为默认值时,运行迁移时会发生同样的崩溃。

我的迁移:

class AddJsonExampleToMyModel < ActiveRecord::Migration
  def change
    add_column :my_models, 
               :json_example
               :json,
               array: true,
               default: []
  end
end

错误看起来像:

PG::InvalidTextRepresentation: ERROR:  malformed array literal: "[]"
DETAIL:  "[" must introduce explicitly-specified array dimensions.
: ALTER TABLE "my_models" ADD COLUMN "json_example" json[] DEFAULT   '[]'/.../db/migrate/20151013125334_add_json_example_to_my_model.rb:3:in     `change'
ActiveRecord::StatementInvalid: PG::InvalidTextRepresentation: ERROR: malformed array literal: "[]"
DETAIL:  "[" must introduce explicitly-specified array dimensions.
: ALTER TABLE "my_models" ADD COLUMN "json_example" json[] DEFAULT '[]'
/.../db/migrate/20151013125334_add_json_example_to_my_model.rb:3:in`change'
PG::InvalidTextRepresentation: ERROR:  malformed array literal: "[]"
DETAIL:  "[" must introduce explicitly-specified array dimensions.

我是在尝试做一些无法完成的事情,还是我做错了?

我的设置:Rails 4.1.9、(PostgreSQL)9.4.4、Ruby 2.1.0p0

解答

Rails 试图将整个值序列化为 JSON 数组,而 Postgres 期望包含 JSON 的本机数组。我建议避免使用 Postgres 数组并存储单个 JSON 对象,它当然可以是一个数组。 Rails 4.1.9 能够正确地序列化它。

这适用于我在 Ruby 2.1.0p0、Rails 4.1.9、Postgres 9.4.4 中:

add_column :things, :data, :json, default: []

> t = Thing.new
 => #<Thing id: nil, data: []>
> t.data = [{a: 1}]
 => [{:a=>1}]
> t.save
   (0.2ms)  BEGIN
  SQL (0.7ms)  INSERT INTO "things" ("data") VALUES ($1) RETURNING "id"  [["data", "[{\"a\":1}]"]]
   (0.5ms)  COMMIT
 => true
> Thing.first
  Thing Load (1.0ms)  SELECT  "things".* FROM "things"   ORDER BY "things"."id" ASC LIMIT 1
 => #<Thing id: 1, data: [{"a"=>1}]>

请注意,如果您在原地修改数组,Rails 也无法看到数组的更改。任何值都是如此,但数组更频繁地以这种方式操作。例如:

> t = Thing.create
   (0.2ms)  BEGIN
  SQL (0.4ms)  INSERT INTO "things" DEFAULT VALUES RETURNING "id"
   (4.7ms)  COMMIT
 => #<Thing id: 2, data: []>
> t.data << 1
 => [1]
> t.save
   (0.2ms)  BEGIN
   (0.2ms)  COMMIT
 => true
> t.reload.data
  Thing Load (0.3ms)  SELECT  "things".* FROM "things"  WHERE "things"."id" = $1 LIMIT 1  [["id", 2]]
 => []

save无事可做,因为它与加载的数组对象相同,即使我们添加了一个值。解决方法是创建新的数组对象或使用*_will_change!告诉 Rails 该值是脏的。

> t.data << 1
 => [1]
> t.data_will_change!
 => [1]
> t.save
   (0.2ms)  BEGIN
  SQL (0.4ms)  UPDATE "things" SET "data" = $1 WHERE "things"."id" = 2  [["data", "[1]"]]
   (0.4ms)  COMMIT
 => true
> t.reload.data
  Thing Load (0.4ms)  SELECT  "things".* FROM "things"  WHERE "things"."id" = $1 LIMIT 1  [["id", 2]]
 => [1]