Experimental/json serializable columns

[jwoertink]

Fixes #691

This is an experimental draft to serializing json columns in to objects. In order to do this, it requires a patch to how the PG shard decodes JSON columns. @matthewmcgarvey submitted will/crystal-pg#232 as a means to fix that hack.

To use them, your migrations would implement the JSON::Any as they do now, so nothing changes there. This means existing columns don't need any sort of migration. In your model, you can continue to use JSON::Any as is without changing anything. If you want to use the serialized column, you'll change the type from JSON::Any to your serializable object type, and then add the column option serialize: true.

struct UserPreferences
  include JSON::Serializable
  property theme : String = "dark"
  property email_notify : Bool = false
end

class User < BaseModel
  table do
    column preferences : UserPreferences, serialize: true
    column media : JSON::Any
  end
end

user = UserQuery.find(4)
user.preferences.theme == "dark"
user.media["image"]?.try(&.as_s)

There's still a few questions left to answer:

• How does the operation handle a serialized column coming in from params?
• What if your column is nullable, but the serialized object has no nilable properties?
• Can we support more complex structures?
• Should there be some sort of runtime escape hatch to get the raw value from the DB of a serialized object?
• Can we provide type-safe compile-time error handling when things aren't setup correctly for this?

[jwoertink]

These had to be moved up so our patch to "pg" comes after the definition

[jkthorne]

I am excited for this feature. I have been using json more and more these days.

[jkthorne]

I think there might also be some query features we can add.

[jwoertink]

@wontruefree Have any ideas for an API for query stuff using this? I know one that I'd like to do is some sort of "has_key?" type method. The json query stuff in postgres is pretty gnarly. Also related: #197 We could add this in and do a separate PR unless it's something minimal...

[paulcsmith]

Lgtm! This is a solid setup to play around with and see how it works