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This is a friendly fork of reifyhealth/specmonstah, with some small alterations to how it behaves (see the CHANGELOG). It is not a better version, it is just a little different. You should probably use the official Reify Health version unless you have a specific reason to use this version.

lambdaisland/specmonstah-malli defaults to this version, but you can exclude it and add an explicit dependency on reifyhealth/specmonstah if you prefer, their APIs are 100% compatible, but the generated data will look a little different.



[lambdaisland/specmonstah "2.0.0"]


Specmonstah (Boston for "Specmonster") lets you write test fixtures that are clear, concise, and easy to maintain. It's great for dramatically reducing test boilerplate.

Say you want to test a scenario where a forum post has gotten three likes by three different users. You'd first have to create a hierarchy of records for the post, topic, topic category, and users. You have to make sure that all the foreign keys are correct (e.g. the post's :topic-id is set to the topic's :id) and that everything is inserted in the right order.

With Specmonstah, all you have to do is write code like this:

(insert {:like [[3]]})

and these records get inserted in a database (in the order displayed):

[[:user {:id 1 :username "T2TD3pAB79X5"}]
 [:user {:id 2 :username "ziJ9GnvNMOHcaUz"}]
 [:topic-category {:id 3 :created-by-id 2 :updated-by-id 2}]
 [:topic {:id 6
          :topic-category-id 3
          :title "4juV71q9Ih9eE1"
          :created-by-id 2
          :updated-by-id 2}]
 [:post {:id 10 :topic-id 6 :created-by-id 2 :updated-by-id 2}]
 [:like {:id 14 :post-id 10 :created-by-id 1}]
 [:like {:id 17 :post-id 10 :created-by-id 2}]
 [:user {:id 20 :username "b73Ts5BoO"}]
 [:like {:id 21 :post-id 10 :created-by-id 20}]]

If you like tools that help you write code that's clear, concise, and easy to maintain, then check out the tutorial and learn how to use Specmonstah :)

Short Sweet Example

If you're more of a gimme fun now kind of person, then try out this little interactive example. First, clone Specmonstah:

git clone

Open examples/short-sweet/short_sweet.clj in your favorite editor and start a REPL. I've also included the code below in case for example you don't have access to a REPL because, say, you're in some kind of Taken situation and you only have access to a phone and you're using your precious battery life to go through this README.

The first ~66 lines of code include all the setup necessary for the examples to run, followed by snippets to try out with example output. Definitely play with the snippets 😀 Can you generate multiple todos or todo lists?

(ns short-sweet
  (:require [reifyhealth.specmonstah.core :as sm]
            [reifyhealth.specmonstah.spec-gen :as sg]
            [clojure.spec.alpha :as s]
            [clojure.spec.gen.alpha :as gen]))

;; Begin example setup

;; ---
;; Define specs for our domain entities

;; The ::id should be a positive int, and to generate it we increment
;; the number stored in `id-seq`. This ensures unique ids and produces
;; values that are easier for humans to understand
(def id-seq (atom 0))
(s/def ::id (s/with-gen pos-int? #(gen/fmap (fn [_] (swap! id-seq inc)) (gen/return nil))))
(s/def ::not-empty-string (s/and string? not-empty #(< (count %) 10)))

(s/def ::username ::not-empty-string)
(s/def ::user (s/keys :req-un [::id ::username]))

(s/def ::created-by-id ::id)
(s/def ::content ::not-empty-string)
(s/def ::post (s/keys :req-un [::id ::created-by-id ::content]))

(s/def ::post-id ::id)
(s/def ::like (s/keys :req-un [::id ::post-id ::created-by-id]))

;; ---
;; The schema defines specmonstah `ent-types`, which roughly
;; correspond to db tables. It also defines the `:spec` for generting
;; ents of that type, and defines ent `relations` that specify how
;; ents reference each other
(def schema
  {:user {:prefix :u
          :spec   ::user}
   :post {:prefix    :p
          :spec      ::post
          :relations {:created-by-id [:user :id]}}
   :like {:prefix      :l
          :spec        ::like
          :relations   {:post-id       [:post :id]
                        :created-by-id [:user :id]}
          :constraints {:created-by-id #{:uniq}}}})

;; Our "db" is a vector of inserted records we can use to show that
;; entities are inserted in the correct order
(def mock-db (atom []))

(defn insert*
  "Simulates inserting records in a db by conjing values onto an
  atom. ent-tye is `:user`, `:post`, or `:like`, corresponding to the
  keys in the schema. `spec-gen` is the map generated by clojure.spec"
  [{:keys [data] :as db} {:keys [ent-type spec-gen]}]
  (swap! mock-db conj [ent-type spec-gen]))

(defn insert [query]
  (reset! id-seq 0)
  (reset! mock-db [])
  (-> (sg/ent-db-spec-gen {:schema schema} query)
      (sm/visit-ents-once :inserted-data insert*))
  ;; normally you'd return the expression above, but return nil for
  ;; the example to not produce overwhelming output

;; Begin snippets to try in REPL

;; Return a map of user entities and their spec-generated data
(-> (sg/ent-db-spec-gen {:schema schema} {:user [[3]]})
    (sm/attr-map :spec-gen))

;; You can specify a username and id
(-> (sg/ent-db-spec-gen {:schema schema} {:user [[1 {:spec-gen {:username "Meeghan"
                                                                :id       100}}]]})
    (sm/attr-map :spec-gen))

;; Generating a post generates the user the post belongs to, with
;; foreign keys correct
(-> (sg/ent-db-spec-gen {:schema schema} {:post [[1]]})
    (sm/attr-map :spec-gen))

;; Generating a like also generates a post and user
(-> (sg/ent-db-spec-gen {:schema schema} {:like [[1]]})
    (sm/attr-map :spec-gen))

;; The `insert` function shows that records are inserted into the
;; simulate "database" (`mock-db`) in correct dependency order:
(insert {:like [[1]]})


This is meant as a quick reference. If none of the terms below make sense, check out the tutorial.

In Specmonstah, you add ents to an ent db using a schema and query. You associate ents with attributes (and perform side effects like db insertion) using visiting functions.


A schema is a map of ent types to ent type schemas:

;; example schema
(def schema
  {:user {:prefix :u}
   :post {:prefix :p}
   :like {:prefix      :l
          :spec        ::like
          :relations   {:post-id       [:post :id]
                        :created-by-id [:user :id]}
          :constraints {:created-by-id #{:uniq}}}})
  • Every ent type schema must have a :prefix key. This is used to name the ents Specmonstah generates.
  • :spec is used by the reifyhealth.specmonstah.spec-gen/spec-gen visiting function to generate values for ents using clojure.spec
  • :relations specify how ents of different types reference each other
  • :constraints provide additional rules around ent generation and visitation:
    • :uniq means that every generated ent must reference a unique ent of the given type. In the schema above, multiple :likes must each reference a distinct :user.
    • :coll indicates that the given attribute can reference multiple ents. See the tutorial
    • :required is used to indicate ent sort order when your ent graph has a cycle

You can also add arbitrary keys to the schema matching the visit-keys you give to visiting functions. The schema will be available to the visiting function under the key schema-opts.


You specify ents to add to an ent db using a query:

(sm/add-ents {:schema schema} {:like [[3]]})

Above, {:like [[3]]} is a query meaning "Add 3 likes to the ent db, as well as the hierarchy of ents necessary for 3 likes to be present."

When you add ents to the ent db, that means that Specmonstah has created a graph node to represent the ent and added it an internal graph that represents all their ents and their relationships.

Visiting functions

You can apply a function to each ent's graph node in topologically sorted (topsort) order and associate the return value as a node attribute.

(Topsort means that if a :post references a :user, then the :user will be placed before the :post in the sort.)

(-> (sm/add-ents {:schema good-schema} {:like [[3]]})
    (sm/visit-ents :prn (fn [db {:keys [ent-name ent-type]}]
                          (prn [ent-name ent-type]))))
[:u1 :user]
[:p0 :post]
[:l1 :like]
[:u0 :user]
[:l0 :like]
[:u2 :user]
[:l2 :like]

In the example above, sm/visit-ents is used to apply an anonymous function to every ent, printing the ent's name and type. The :prn key is called the visit key. The return value of the visiting function is associated with each ent node using the visit key.

The first argument to the visit function is always the entire ent db. The second argument is a map that includes the following keys:

  • :ent-name: :u0, :u1 and the like
  • :attrs: a map of all node attrs for the ent. These attrs are also merged into the map passed to the visit function
  • :visit-val - current value of the visit attr for this node. Could be present from previous visits.
  • :visit-key, the key used to associate the return value of the visit fn with the node
  • :query-opts: any options you might have included in the query used to generate this node
  • :visit-query-opts: just looks up the value of :visit-key in the :query-opts map
  • :schema-opts: any options set for :visit-key in the schema

Can you improve this documentation? These fine people already did:
Daniel Higginbotham, Natxo Cabré, khiemlam, David Chelimsky, Andrea Crotti, Arne Brasseur, Arsene Rei, Christian Romney, Jakub Holy, Taylor Wood & Alan Dipert
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