# seql

**Repository Path**: mirrors_exoscale/seql

## Basic Information

- **Project Name**: seql
- **Description**: Simplfied EDN Query Language for SQL
- **Primary Language**: Unknown
- **License**: ISC
- **Default Branch**: master
- **Homepage**: None
- **GVP Project**: No

## Statistics

- **Stars**: 0
- **Forks**: 0
- **Created**: 2020-09-24
- **Last Updated**: 2026-05-23

## Categories & Tags

**Categories**: Uncategorized

**Tags**: None

## README

seql: Simplified EDN Query Language
===================================

**seql** intends to provide a simplified
[EQL](https://edn-query-language.org/) inspired query language to
access entities stored in traditional SQL databases.
![](https://github.com/exoscale/seql/workflows/Clojure/badge.svg)
[![cljdoc badge](https://cljdoc.org/badge/exoscale/seql)](https://cljdoc.org/d/exoscale/seql/CURRENT)
[![Clojars Project](https://img.shields.io/clojars/v/exoscale/seql.svg)](https://clojars.org/exoscale/seql)

## Introduction

Accessing SQL entities is often done based on a pre-existing
schema. In most designs, applications strive to limit the number of
ways mutations should happen on SQL. However, queries often need to be
very flexible in the type of data they return as well in the number of
joins performed.

With this rationale in mind, **seql** was built to provide:

- A data-based schema syntax to describe entities stored in SQL, as
  well as their relations to each other, making no assumptions on the
  database layout
- A mapping between maps described with clojure.spec and records stored
  in databases with implicit coercion support.
- A subset of the schema dedicated to expressing mutations and their
  input to allow for validation at the edge
- A query builder allowing ad-hoc relations to be expressed
- A mutation handler
- A *listener* facility to support CQRS approaches

**seql** relies on three key libraries:

- [next.jdbc](https://github.com/seancorfield/next.jdbc) to provide SQL access
- [honeysql](https://github.com/seancorfield/honeysql) to build SQL queries from data
- [coax](https://github.com/exoscale/coax) for coercion of records

Where applicable, these dependencies are made obvious.

## Changelog

### 0.2.2

- Deeper spec integration
- Idents and conditions are now infered

### 0.1.29

- Upgrade to recent `next.jdbc` and `honeysql`

### 0.1.2

Minor fixes for errors spotted by eastwood.

### 0.1.1

Inital release.

## Thanks

This project was greatly inspired by @wilkerlucio's work on
[pathom](https://github.com/wilkerlucio/pathom) and subsequently
[EQL](https://github.com/edn-query-language/eql).  As a first consumer
of this library @davidjacot also helped iron out a few kinks and made
some significant improvements. The 0.2.x release benefited from the
sound advice and reviews of @arnaudgeiser and @mpenet.

## Quickstart

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

![schema](https://i.imgur.com/DkBtyew.png)

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:

```clojure
(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 entity-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:

``` clojure
(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:

```clojure
(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:

```clojure
(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:

```clojure
(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:

``` clojure
(make-schema
  (entity-from-spec ::account/account))
```

We can now perform the following query:

``` clojure
(query env ::account/account [::account/name] [[::account/state :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:

```clojure
(make-schema
  (entity-from-spec ::account/account
    (has-many ::users [:id ::user/account-id]))

  (entity-from-spec ::user/user))
```

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

```clojure
(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 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:

```clojure
(make-schema
 (entity-from-spec ::account/account
            (has-many :users    [:id ::user/account-id])
            (has-many :invoices [:id ::invoice/account-id]))
 (entity-from-spec ::user/user)
 (entity-from-spec ::invoice/invoice
            (has-many :lines    [:id ::invoice-line/invoice-id]))
 (entity-from-spec ::product/product)
 (entity-from-spec [::invoice-line/invoice-line :invoiceline]
            (has-one :product [:product-id ::product/id])))
```

### Controlling the mapping betwen row and column names in the database

Specific table names can be provided by using a vector as the argument
for `entity` or `entity-from-spec`:

``` clojure
(make-schema
  (entity-from-spec [::invoice-line/invoice-line :invoiceline]
    ...))
```

Specific column names can be provided by using the `column-name` helper:

``` clojure
(make-schema
  (entity-from-spec ::network/network
    (column-name :ip6address :ip6)
    ...))
```

### Mutations

With querying sorted, mutations need to be expressed. Here, **seql**
takes the approach of making mutations separate, explictit, 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.

For the common case of inserting, updating, or deleting records from the database,
a couple of schema helpers are provided.


#### Inserting records with `add-create-mutation`

To allow record insertion, use the `add-create-mutation` helper:

```clojure
 (entity-from-spec ::account/account
            (has-many :users    [:id ::user/account-id])
            (has-many :invoices [:id ::invoice/account-id])
            (add-create-mutation))
```

The implicit mutation created by `add-create-mutation` will be
named: `::account/create`, a spec has to exist for it, as for all
mutations. Since `spec/valid?` runs on input parameters before handing
out to mutation functions it should always be present (otherwise mutations
will throw early).

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

Adding new accounts can now be done through `mutate!`:

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

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

;; =>

#::account{:state :active}
```

#### Updating records with `add-update-by-id-mutation`

To allow record updates, use the `add-update-by-id-mutation` helper:

``` clojure
 (entity-from-spec ::account/account
            (has-many :users    [:id ::user/account-id])
            (has-many :invoices [:id ::invoice/account-id])
            (add-create-mutation)
            (add-update-by-id-mutation ::account/id))
```

This instructs the helper that the input map to the mutation function
will contain a `::account/id` field which should be used to determine
which row to update in the database. The rest of the map contents will
be treated as values to update in the database.

#### Deleting records with `add-delete-by-id-mutation`

To allow record deletes, use the `add-delete-by-id-mutation` helper:

``` clojure
 (entity-from-spec ::account/account
            (has-many :users    [:id ::user/account-id])
            (has-many :invoices [:id ::invoice/account-id])
            (add-create-mutation)
            (add-update-by-id-mutation ::account/id)
            (add-delete-by-id-mutation ::account/id))
```
This instructs the helper that the input map to the mutation function
will contain a `::account/id` field which should be used to determine
which row to delete from the database.

#### Arbitrary mutations with `mutation-fn`

It is hard to predict all types of mutations, and often times, any such attempt
results in worse ergonomics than what SQL provideds. To this end, `seql` allows
providing arbitrary SQL expressions as mutations through the help of `honeysql`

``` clojure
(entity-from-spec ::account/account
 (has-many :users    [:id ::user/account-id])
 (has-many :invoices [:id ::invoice/account-id])
 (mutation-fn :remove-users (s/keys :req [::account/id])
     (fn [params] {:delete-from [:users] :where [:= :account-id (::account/id params)]})))
```

#### Mutation preconditions

Mutations can be provided with preconditions: functions to run before affecting the actual
mutation. These run in the same transaction as the effective mutation.

``` clojure
(entity-from-spec ::account/account
 (add-create-mutation)
 (add-update-by-id-mutation ::account/id)
 (add-precondition :delete ::has-no-users?
   (fn [{::account/keys [id]}]
     ;; Needs to go through HoneySQL
     {:select [:id] :from [:users] :where [:= :account-id id]})
   ;; Ensure the result is empty
   empty?))
```

#### Transactions over several mutations

Mutations can be performed in a larger transaction cycle. To this effect, the
`seql.mutation/with-transaction` macro is provided:

``` clojure
(m/with-transaction env
  (m/mutate! env ::account/create account-a)
  (m/mutate! env ::user/create user1-in-account-a)
  (m/mutate! env ::user/create user2-in-account-a)
  (q/execute env [::account/id (::account/id account-a]]))
```

### 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

```clojure
(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}
```