kemal mysql

mess.cr

require "./mess/*"
require "kemal"
require "json"
require "mysql"
require "pool/connection"

db = ConnectionPool.new(capacity: 25, timeout: 0.01) do
  DB.open(ENV["DATABASE_URL"])
end

class User
  DB.mapping({
    id: Int32,
    first_name:   {type: String, nilable: true},
    surname: {type: String, nilable: true},
  })
  JSON.mapping({
    id: Int32,
    first_name:   {type: String, nilable: true},
    surname:  {type: String, nilable: true},
  })
end

get "/" do |env|
  db.connection do |conn|
    env.response.content_type = "application/json"
    rs = conn.query("SELECT id,first_name,surname FROM users")
    users = User.from_rs(rs)
    rs.close #must do this to allow the connection to be reused
    return users.to_json
  end
end

Kemal.run

shard.yml

name: mess
version: 0.1.0

authors:
  - Cris Ward 

license: MIT

dependencies:
  kemal:
    github: sdogruyol/kemal
    branch: master
  mysql:
    github: crystal-lang/crystal-mysql
    branch: master
  pool:
    github: ysbaddaden/pool
    branch: master

for followers, DB.mapping is described here https://github.com/crystal-lang/crystal-db/blob/4f724475e0b929fe028186e7bce8a656fcb4f46d/src/db/mapping.cr and "from_rs" means "from result set" :|
You might be able to avoid having to call close on the resultset by putting it in a block.