11: collection constraint (vector of foreign keys)

Let's say the forum you're building stores a user's favorite posts as a vector of foreign keys, like this:
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{:username "birb"
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 :favorite-ids [1 2 3 4]}
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Here's how you'd model this in Specmonstah:
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(ns reifyhealth.specmonstah-tutorial.11
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  (:require [clojure.spec.alpha :as s]
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            [reifyhealth.specmonstah.core :as sm]
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            [reifyhealth.specmonstah.spec-gen :as sg]))
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​
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(s/def ::id (s/and pos-int? #(< % 100)))
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​
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(s/def ::post (s/keys :req-un [::id]))
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​
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(s/def ::favorite-ids (s/coll-of ::id))
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(s/def ::user (s/keys :req-un [::id ::favorite-ids]))
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​
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(def schema
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  {:post {:prefix :p
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          :spec   ::post}
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   :user {:prefix      :u
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          :spec        ::user
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          :relations   {:favorite-ids [:post :id]}
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          :constraints {:favorite-ids #{:coll}}}})
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There's our buddy :constraints again :) This time the constraint is :coll. This is used by the clojure.spec visitor to generate data correctly:
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(defn ex-01
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  []
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  (sg/ent-db-spec-gen-attr {:schema schema} {:user [[1]]}))
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​
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(ex-01) ;; =>
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{:p0 {:id 6}
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 :u0 {:id 9, :favorite-ids [6]}}
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Specmonstah generated a post with :id 6, and sure enough you can see that the value of :favorite-ids is a vector containing the post's :id. But what if you want to generate multiple posts? Here's how you'd do that:
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(defn ex-02
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  []
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  (sg/ent-db-spec-gen-attr {:schema schema}
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                           {:user [[1 {:refs {:favorite-ids 3}}]]}))
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(ex-02)
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;; =>
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{:p0 {:id 6}
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 :p1 {:id 21}
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 :p2 {:id 8}
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 :u0 {:id 1
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      :favorite-ids [6 21 8]}}
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And boom goes the dynamite. The key here is the map {:refs {:favorite-ids 3}} - this tells Specmonstah that this user will have 3 :favorite-ids.
You might be wondering, what happens if you generate 2 separate users, each with 3 :favorite-ids? Well my friend, wonder no more:
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(defn ex-03
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  []
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  (sm/view (sm/add-ents {:schema schema}
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                        {:user [[2 {:refs {:favorite-ids 3}}]]})))
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(ex-03)
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Both users reference the same posts because Specmonstah generates the minimum data necessary to satisfy the query. You can also "manually" specify the ents you want to reference:
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(defn ex-04
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  []
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  (sm/view (sm/add-ents {:schema schema}
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                        {:user [[1 {:refs {:favorite-ids [:my-p0 :my-p1]}}]
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                                [1 {:refs {:favorite-ids [:my-p2 :my-p3]}}]]})))
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(ex-04)
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10: uniqueness constraint

12: what about cycles?

Last modified 2yr ago

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