Allow multiple instances of datasources about gungnir HOT 2 CLOSED

;; db1.clj
(ns myproj.db1 (:require [gungnir.database :as gungnir]))

;;  calling make-datasource returns a state-map
;;  this state map has all the functions and state you need to operate with this particular connection
(let [{:keys [close-fn changeset-fn datasource find!-fn find-by!-fn all!-fn save!-fn]}
      (gungnir/make-datasource! :datasource1
                                {:adapter "postgresql" :username ....})]

  ;;  the idea for this was taken from the well known and mature websockets library sente
  ;;  see the first code example in the sente readme https://github.com/ptaoussanis/sente
  ;;  the tradeoff is this little bit of boilerplate here
  ;;  but this could easily be wrapped into a gungnir provided namespace for the default case
  (def *datasource* datasource)
  (def find! find!-fn)
  (def find-by! find-by!-fn)
  (def all! all!-fn)
  (def save! save!-fn)
  (def changeset changeset-fn)
  (def close close-fn))


;; now an example of how to use it
;; user.clj
(ns myproj.user
  (:require [myproj.db1 :refer [find! find-by! all! changeset save!]]
            [gungnir.model :as model]))

(model/register!
  {:user
    [:map
     [:user/id {:primary-key true} uuid?]
     [:user/email [:re {:error/message "Invalid email"} #".+@.+\..+"]]
     [:user/password [:string {:min 6}]]
     [:user/created-at {:auto true} inst?]
     [:user/updated-at {:auto true} inst?]]})

(defn by-id [id] (find! id))
(defn by-email [email] (find-by! email))
(defn change-password [id new-password]
  (save! (changeset (by-id id) {:user/password new-password})))

;; that was the basic use case
;; now, how would one connect to multiple datasources?
;; well, we duplicate db1, but as db2, so you have myproj.db1 and myproj.db2
;; each should connect to different databases, but def the same functions

;; product.clj
;; in this example the user database is separate from the database of products
(ns myproj.products
  (:require [myproj.db2 :as db2]
            [gungnir.model :as model]))

(model/register!
  {:product
    [:map
     [:product/id {:primary-key true} uuid?]
     [:product/sku [:re {:error/message "Invalid SKY"} #"SKU-.*"]]
     [:product/name [:string]]
     [:product/amount-in-stock [:int {:min 0}]]
     [:product/description [:string]]
     [:product/image [:string]]
     [:product/created-at {:auto true} inst?]
     [:product/updated-at {:auto true} inst?]]})

(defn by-id [id] (db2/find! id))
(defn by-sku [sku] (db2/find-by! sku))
(defn decrement-stock [sku]
  (let [old-p (by-sku sku)]
    (db2/save! (db2/changeset old-p (update old-p :product/amount-in-stock dec)))))