Quickstart

Let us assume the following - admittedly flawed - schema, for which we will add gradual support:

All the following examples can be reproduced in the test/seql/readme_test.clj integration test. To perform queries, an environment must be supplied, which consists of a schema, and a JDBC config. In test/seql/fixtures.clj, code is provided to experiment with an H2 database.

For all schemas displayed below, we assume an env set up in the following manner:

(def env {:schema ... :jdbc your-database-config})

(require '[seql.query :as q])
(require '[seql.lister :as l])
(require '[seql.mutation :as m])
(require '[clojure.spec.alpha :as s])
(require '[seql.helpers :refer [make-schema ident idents field mutation
                                has-many condition from-spec]])

Specs for the schema

Seql assumes you are familiar with clojure.spec if that is not the case, please refer to: https://clojure.org/guides/spec

We can start by providing specs for the individual fields in each table:

(create-ns 'my.entities)
(create-ns 'my.entities.account)
(create-ns 'my.entities.user)
(create-ns 'my.entities.invoice)
(create-ns 'my.entities.invoice-line)
(create-ns 'my.entities.product)


(alias 'account 'my.entities.account)
(alias 'user 'my.entities.user)
(alias 'invoice 'my.entities.invoice)
(alias 'invoice-line 'my.entities.invoice-line)
(alias 'product 'my.entities.product)

(s/def ::account/name string?)
(s/def ::account/state #{:active :suspended :terminated})
(s/def ::account/account (s/keys :req [::account/name ::account/state]))

(s/def ::user/name string?)
(s/def ::user/email string?)
(s/def ::user/user (s/keys :req [::user/name ::user/email]))

(s/def ::invoice/state keyword?)
(s/def ::invoice/total nat-int?)
(s/def ::invoice/invoice (s/keys :req [::invoice/state ::invoice/total]))

(s/def ::invoice-line/quantity nat-int?)
(s/def ::invoice-line/invoice-line (s/keys :req [::invoice-line/quantity]))

(s/def ::product/name string?)
(s/def ::product/product (s/keys :req [::product/name]))

Queries on accounts

At first, accounts need to be looked up. We can build a minimal schema:

(make-schema
  (entity ::account/account
		  (field :name)
		  (field :state)))

Let's unpack things here:

• We give a name our entity, by default it will be assumed that the SQL table it resides in is eponymous, when it is not the case, a tuple of [entity-name table-name] can be provided
• We declare a list of fields known to exist in that table.

With this, simple queries can be performed:

(query env ::account/account [::account/name ::account/state])
;; or to fetch all default fields:
(query env ::account/account)

;; =>

[#::account{:name "a0" :state :active}
 #::account{:name "a1" :state :active}
 #::account{:name "a2" :state :suspended}]

Idents can also be looked up:

(query env [::account/id 0] [::account/name ::account/state])

;; =>

#::account{:name "a0" :state :active}

Notice how the last query yielded a single value instead of a collection. It is expected that idents will yield at most a single value (as a corollary, idents should only be used for database fields which enforce this guarantee).

Also notice how there was no prior mention of ::account/id

Infering schemas from specs

A first concrete improvement we can bring to the schema build step when an s/keys spec is available for our entity is to infer most of the schema from it:

(make-schema
  (from-spec ::account/account))

While here, we can also add a short name to provide a shortcut condition for [::account/state :active]:

(make-schema
  (from-spec ::account/account
    (condition :active :state :active)
    (condition :state)))

We can now perform the following query:

(query env ::account/account [::account/name] [[::account/active]])

;; =>

[#::account{:name "a0"}
 #::account{:name "a1"}]


(query env ::account/account [::account/name] [[::account/state :suspended]])

;; =>

[#::account{:name "a2"}]

Adding a relation

For queries, seql's strength lies in its ability to understand the way entities are tied together. Seql offers support for one-to-many (has many), one-to-one (has one), and many-to-many (has many through) relations.

Let's start with a single relation before building larger nested trees. Since no assumption is made on schemas, the relations must specify foreign keys explictly:

(make-schema
  (from-spec ::account/account
    (has-many ::users [:id ::user/account-id])
    (condition :active :state :active))

  (from-spec ::user/user))

This will allow doing tree lookups, fetching arbitrary fields from the nested entity as well:

(query env
       ::account/account
       [::account/name
        ::account/state
        {::account/users [::user/name ::user/email]}])

;; =>

[#::account{:name  "a0"
            :state :active
            :users [#::user{:name "u0a0" :email "u0@a0"}
                    #::user{:name "u1a0" :email "u1@a0"}]}
 #::account{:name  "a1"
            :state :active
            :users [#::user{:name "u2a1" :email "u2@a1"}
                    #::user{:name "u3a1" :email "u3@a1"}]}
 #::account{:name "a2" :state :suspended}]

Summary of query description

We've now covered full capabilities of the query part of the schema, were we saw that:

• Each entity should at least have a table.
• To provide more idiomatic output, spec based coercions are performed in and out of the database.
• Conditions allow for building advanced filters on entities.
• To build arbitrarily nested entities, relations need to be used.

With this in mind, here's a complete schema for the above database schema:

(make-schema
 (from-spec ::account/account
            (has-many :users    [:id ::user/account-id])
            (has-many :invoices [:id ::invoice/account-id])
            (condition :active  :state :active)
            (add-create-mutation))
            (mutation :account/update
                      ::account
                      [{:keys [id] :as params}]
                      (-> (h/update :account)
                          ;; values are fed unqualified
                          (h/set (dissoc params :id))
                          (h/where [:= :id id]))))
 (from-spec ::user/user)
 (from-spec ::invoice/invoice
            (has-many :lines    [:id ::invoice-line/invoice-id])
            (condition :unpaid  :state :unpaid)
            (condition :paid    :state :paid))
 (from-spec ::product/product)
 (from-spec [::invoice-line/invoice-line :invoiceline]
            (has-one :product [:product-id ::product/id])))

Mutations

With querying sorted, mutations need to be expressed. Here, seql takes the approach of making mutations separate, explict, and validated. As with most other seql features, mutations are implemented with a key inside the entity description.

At its core, mutations expect two things:

• A spec of their input
• A function of this input which must yield a proper honeysql query map, or collection of honeysql query map to be performed in a transaction.

To simplify matters, add-create-mutation can be used for inserts.

 (from-spec ::account/account
            (has-many :users    [:id ::user/account-id])
            (has-many :invoices [:id ::invoice/account-id])
            (condition :active  :state :active)
            (add-create-mutation))
            (mutation :account/update
                      ::account
                      [{:keys [id] :as params}]
                      (-> (h/update :account)
                          ;; values are fed unqualified
                          (h/set (dissoc params :id))
                          (h/where [:= :id id]))))

The implicit mutation created by add-create-mutation will be named: ::account/create, a spec has to exist for it:

(s/def ::account/create ::account/account)

Adding new accounts can now be done through mutate!:

(mutate! env ::account/create {::account/name  "a3"
                               ::account/state :active})

(query env [::account/name "a3"] [::account/state])

;; =>

#::account{:state :active}

Listeners

To provide for clean CQRS type workflows, listeners can be added to mutations. Each listener will subsequently be called on sucessful transactions with a map of:

• :mutation: the name of the mutation called
• :result: the result of the transaction
• :params: input parameters given to the mutation
• :metadata: metadata supplied to the mutation, if any

(def last-result (atom nil))

(defn store-result
  [details]
  (reset! last-result (select-keys details [:mutation :result])))

(let [env (l/add-listener env ::account/create store-result)]
   (mutate! env ::account/create {::account/name "a4"
                                  ::account/state :active}))

@last-result

;; => {:result [1] :mutation :account/create}