[guide] add a section for JSON serialization with Dry::Types::Constructor

source/4.0/guides/index.html.md

@@ -2,6 +2,7 @@
 title: Guides
 sections:
   - sql-quick-start
+  - sql-how-to
 ---
 
 Guides are a collection of short articles explaining how to achieve common tasks

source/4.0/guides/sql-how-to/index.html.md

@@ -0,0 +1,8 @@
+---
+chapter: SQL How To
+sections:
+- store-data-json-formatting
+---
+
+In this section you can learn some tricks with ROM and SQL.
+

source/4.0/guides/sql-how-to/store-data-json-formatting.html.md

@@ -0,0 +1,51 @@
+---
+chapter: SQL How To
+title: Store data as JSON Strings
+---
+
+Let's assume that you are using a storage system like MySQL and you want to store some data in JSON format. Let's also
+assume that you don't have Hash kind like Type provided by default by this storage system (like HStore on Postgres,
+in such case you can use [built-in PG types](https://rom-rb.org/4.0/learn/sql/schemas/#postgresql-types)).
+
+You can use `Dry::Types:Constructor` in order to make your own way to process data, and give it to your ROM::Relation 
+in order that ROM uses this to store and read this data.
+
+You can use same technique if you need any data processing before it goes to your Datastore (like YAML serialization).
+
+```ruby
+JSONRead = Dry::Types::Constructor.new(Dry::Types['string'], fn: ->(v) { JSON.parse(v) })
+JSONWrite = Dry::Types::Constructor.new(Dry::Types['string'], fn: ->(v) { JSON.dump(v) })
+```
+Those `Dry::Types::Constructor` defines the way to translate input object to String.
+
+You can test those by doing this:
+```ruby
+h = {'a'=>'a', 'b'=>{ 'c'=>'c'}}
+JSONWrite[h]
+=> "{\"a\":\"a\",\"b\":{\"c\":\"c\"}}"
+
+JSONRead[JSONWrite[h]]
+=> {"a"=>"a", "b"=>{"c"=>"c"}}
+
+JSONRead[JSONWrite[h]] == h
+=> true
+```
+Tips: You can use [Oj gem](https://github.com/ohler55/oj) if you want to keep symbol keys when serializing JSON.
+
+Then on your relations:
+```ruby
+module Relations
+  class Users < ROM::Relation[:sql]
+    schema(:users) do
+      attribute :hash_attribute, JSONWrite, read: JSONRead
+      attribute :array_attribute, JSONWrite, read: JSONRead
+    end
+  end
+end
+```
+Here, we assume that you're using SQL datastore, with a `users` table that has `hash_attribute` column which is a String
+and a `array_attribute` column that is also a String.
+
+Now you'll have ruby Hash on your ROM::Struct automatically instead of Strings in JSON format.
+
+This article as been inspired by this [forum post](https://discourse.rom-rb.org/t/methods-of-storing-arrays-in-mysql/305).