On Sat, 15 Dec 2012, mond wrote:
Thanks for picking up the cudgels Ben!
Ha. It's nice to have reached a point where I feel at-all confident
with any of this... happy to help.
To be honest I am struggling to repeat your advice in the REPL. In
any case, I decided to change the data structures in line with your
advice and put the IDs into maps rather than the records.
(defrecord Item [name product quantity purchased])
(defrecord Product [name description prices])
(defrecord Price [price tax currency])
(def items [ {:id 1 :item (->Item "Brogues" "P-123" 1 true)}
{:id 2 :item (->Item "Underpants" "P-345" 2 false)}
{:id 3 :item (->Item "Shirt" "P-678" 1 true)} ])
(def carts [ (->Cart "Birthday" (first items))
(->Cart "Xmas" (rest items)) ])
(def products [ {:id "P-1231" :product (->Product "Table" "Coffee Table" (->Price 375 21
"EURO"))}
{:id "P-3451" :product (->Product "Chairs" "Set of Four(4) chairs"
(->Price 200 21 "EURO"))}
{:id "P-123" :product (->Product "Brogues" "Men's Leather Slip on Brogues"
(->Price 93.00 21 "EURO"))}
{:id "P-345" :product (->Product "Underpants" "CK Y Fronts" (->Price
23.50 21 "EURO"))}
{:id "P-678" :product (->Product "Shirt" "Slim Fit White Vest Shirt"
(->Price 45.99 21 "EURO"))}
{:id "P-6781" :product (->Product "TableCloth" "Classic red and white checks 2m x
2m" (->Price 17.99 21 "EURO"))} ])
Do you think the zipmap is still the way to go (to resolve the
'foreign key' or could there be an easier way? I guess the fact that
only items have to zipmapped is one advantage.
It seems like items and products should still have an :id property, so I
don't think you need to detach the ID from its entity. The zipmap'ed
version is useful as an index, not really as its own structure. So, it
ends up as just a single map (not an array of maps) with ID's as keys,
and the corresponding entity as the value:
(defrecord Item [id name product quantity purchased])
(defrecord Product [id name description prices])
(defrecord Price [price tax currency])
(def items [ (->Item 1 "Brogues" "P-123" 1 true)
(->Item 2 "Underpants" "P-345" 2 false)
(->Item 3 "Shirt" "P-678" 1 true) ])
(def products [ (->Product "P-1231" "Table" "Coffee Table" (->Price 375 21
"EURO"))
(->Product "P-3451" "Chairs" "Set of Four(4) chairs" (->Price 200 21
"EURO"))
(->Product "P-123" "Brogues" "Men's Leather Slip on Brogues" (->Price
93.00 21 "EURO"))
(->Product "P-345" "Underpants" "CK Y Fronts" (->Price 23.50 21
"EURO"))
(->Product "P-678" "Shirt" "Slim Fit White Vest Shirt" (->Price 45.99 21
"EURO"))
(->Product "P-6781" "TableCloth" "Classic red and white checks 2m x 2m"
(->Price 17.99 21 "EURO")) ])
; two useful indexes(/indices?):
(def prod->item
"An index from Product ID to an Item"
(zipmap (map :product items) items))
(def id->product
"An index from Product ID to a Product"
(zipmap (map :id products) products))
; Then you can use that to get the products joined with their items:
(defn product-with-item
"Find the Item info and merge it into the product, without overwriting :id"
[product]
(merge-with (fn [a b] a) product (prod->item (:id product))))
; example usage:
(product-with-item (id->product "P-345"))
;=> #user.Product{:id 2,
; :name "Underpants",
; :description "CK Y Fronts",
; :prices #user.Price{:price 23.5, :tax 21, :currency "EURO"},
; :purchased false,
; :quantity 2,
; :product "P-345"}
(def joined-products (map product-with-item products))
;=> A list of all the products joined with their items
--
Best,
Ben
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