Great stuff Ben. Thanks.
ray
On Saturday, December 15, 2012 10:54:42 PM UTC+1, Benjamin R. Haskell wrote:
>
> 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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