Walkable uses EQL, a Clojure native query language to describe what data you want from your SQL DBMS. The query language was inspired by Datomic's Pull API and Graphql and first introduced in om.next. However, you don't have to use om.next (or its fork, Fulcro) to make use of the query language.

If you know GraphQL, then the main differences between the two query languages are:

  • GraphQL is built atop strings, in constrast with Clojure's rich data structure used by EQL.

  • EQL encourages the use of namespaced (aka qualified) keywords. In fact, you must use namespaced keywords with Walkable so it can infer which column is from which table. For instance, :person/id means the column id in the table person.

  • GraphQL is coupled with a type system. EQL is not.

If you're familiar with building apps with om.next/fulcro, please note: you often specify a query for each component and compose them up. From server-side perspective (which is that of Walkable's), you don't know (or care) about those query fragments: you just return the right data in the right shape for the big final top-level query. The reconciler in the client-side will build up the query and break down the result.

1. Properties

To query for an entity's properties, use a vector of keywords denoting which properties you're asking for.

For example, use this query

;; query of three prop keys:
[:person/id :person/name :person/age]

when you want to receive things like:

;; returned value
{:person/id   99
 :person/name "Alice"
 :person/age  40}

the thing that receives a query and returns the above is called a query resolver (or simply "query parser" in Om.next, Fulcro and Pathom's documentation).

2. Joins

Sometimes you want to include another entity that has some kind of relationship with the current entity.

;; returned value
{:person/friends [{:person/id   97
                   :person/name "Jon"}
                  {:person/id   98
                   :person/name "Mary"}]
 :person/mate    {:person/id   100
                  :person/name "Bob"}
 :person/pets    [{:pet/id   10
                   :pet/name "Nyan cat"}
                  {:pet/id   20
                   :pet/name "Ceiling cat"}
                  {:pet/id   30
                   :pet/name "Invisible Bike Cat"}]}

to achieve that, the query should be:

;; query of three join keys:
[:person/friends :person/mate :person/pets]

which is the same as:

;; query
[{:person/friends [*]}
 {:person/mate    [*]}
 {:person/pets    [*]}]

which means you let the query resolver dictate which child properties to return for each join. Or you may explicitly tell your own list:

;; query
[{:person/friends [:person/id :person/name]}
 {:person/mate    [:person/id :person/name]}
 {:person/pets    [:pet/id    :pet/name]}]

Wait, isn't this list the vector syntax I've learned in Properties section? Yup.

You may also notice that the query syntax is the same for to-many relationships (ie :person/friends and :person/pets) as well as to-one one (:person/mate). Well, the query resolver, which owns the data, will decide if it will return an item (as a map) or a vector of zero or more such item.

3. Idents

Actually the properties and joins above can't stand alone themselves. They must stem from somewhere: enter idents. Idents are the root of all queries (or the root of all evals :D)

I lied in the examples in section 1 and 2: such queries are not enough for the query resolver to return such results. So what's missing? You guess... Idents!

Let's learn about two types of them.

3.1 Keyword idents

Some idents look just like joins:

;; query
[{:my-profile [:person/name :person/age]}]

of course idents can have joins, too:

;; query
[{:my-profile [:person/name :person/age
               {:person/pets [:pet/name :pet/id]}]}]

3.2 Vector idents

Other idents look a bit weird. Instead of being a keyword, they are made of a vector of a keyword indicating the entity type followed by exactly one argument specifying how to identify those entities.

First, look at the ident vector:

;; queries
[:person/by-id 1]
;; or
[:thing/by-uuid "03157713-28f8-4f2b-9aa2-3fc52451369a"]
;; or
[:people-list/by-member-ids #{1 2 3}]

Okay, now see them in context:

;; full queries
[{[:person/by-id 1] [:person/name :person/age]}]

[:person/by-id 1] is called the ident key. [:person/name and :person/age are the child properties.

[{[:people-list/by-member-ids #{1 2 3}] [:person/name :person/age]}]

[:people-list/by-member-ids #{1 2 3}] is the ident. Again, [:person/name and :person/age are the child properties.

Allow yourself some time to grasp the syntax. Once you're comfortable, here is the above queries again with a child join added:

;; queries
[{[:person/by-id 1] [:person/name :person/age
                     {:person/pets [:pet/name :pet/id]}]}]

[{[:people-list/by-member-ids #{1 2 3}] [:person/name :person/age
                                         {:person/pets [:pet/name :pet/id]}]}]

You may notice idents also live inside a vector, which means you can have many of them in your query:

These two queries:

;; queries
[{[:person/by-id 1] [:person/name :person/age]}]
[{[:person/by-id 2] [:person/name :person/age]}]

can be merged into one

;; query
[{[:person/by-id 1] [:person/name :person/age]}
 {[:person/by-id 2] [:person/name :person/age]}]

actually, idents can stem from anywhere, like this:

;; query
[{[:person/by-id 1] [:person/name :person/age
                     {[:person/by-id 2] [:person/name :person/age]}]}]

Here [:person/by-id 2] looks just like a child join (such as :person/mate, :person/pets), but it has nothing to do with the entity [:person/by-id 1].

Todo: sample results from each query above.

4. Key vs dispatch key

After reading about properties vs joins vs idents, you may think: "This doesn't make sense! If an ident can sometimes look like a join, and a join can sometimes look like a property, then what's the point of defining a syntax at all?

Well, the distinction is less in syntax and more in semantics. For each key, the author of query resolver should have decided if it's a property, a join or an ident. Looking at the dispatch key in the key itself is an efficient way to do it.

What's a dispatch key? Let's learn to recognize them through some examples:

key type dispatch-key
:people/all keyword ident :people/all
[:person/by-id 1] vector ident :person/by-id
:person/pets join :human/pets

5. Parameters

Sometimes you want the query resolver to modify a little bit of the data. Parameters are the piece to communicate just that.

Parameters must be implemented from the query resolver's side in order to have effect. The parameters in the examples below are provided to explain the syntax, so you get the idea.

There are two ways to denote parameters, the new and the legacy syntax. Let's go for the new one first.

5.1 New syntax for parameters

Without params
With params
;; simple query
'[:person/name :person/height]
;; vs modified query with params in the property `:person/height`
'[:person/name (:person/height {:unit :cm})]

Just like a Clojure function's list of arguments, parameters may contain zero or more items. Personally, I prefer the use of exactly one hash-map. For instance, with Walkable you can use some pre-defined parameters:

Without params
With params
;; simple query
'[{:people/all [:person/name :person/age]}]
;; vs modified query with params `{:order-by :person/name}` added to the ident `:people/all`
'[{(:people/all {:order-by :person/name}) [:person/name :person/age]}]

Without params
With params
;; simple query
'[{:people/all [:person/name :person/age]}]
;; vs modified query with params `{:offset 20 :limit 10}` added to the ident `:people/all`
[{(:people/all {:offset 20 :limit 10}) [:person/name :person/age]}]

Of course, someone else with a different taste may implement their query resolver to accept keyword parameters instead:

Without params
With params
;; simple query
'[{:people/all [:person/name :person/age]}]
;; vs modified query with params `'(:offset 20 :limit 10)` added to the ident `:people/all`
'[{(:people/all :offset 20 :limit 10) [:person/name :person/age]}]

Without params
With params
;; simple query
'[:person/name :person/age]
;; vs modified query with params `'(:unit :cm)` in the property `:person/height`
'[:person/name (:person/height :unit :cm)]

That's valid syntax, too.

5.2 Legacy syntax for parameters

This section is required only if you're a user of om.next or fulcro older than v2.2.1. Feel free to skip it otherwise.

The syntax for parameters of properties is the same.

For idents and joins, you put the whole query inside a list, followed by the parameters:

Without params
With params
;; simple query
'[{:people/all [:person/name :person/age]}]
;; vs modified query with params `{:order-by :person/name}` added to the ident `:people/all`
'[({:people/all [:person/name :person/age]}
   {:order-by :person/name})]

Without params
With params
;; simple query
'[{:people/all [:person/name :person/age]}]
;; vs modified query with params `{:offset 20 :limit 10}` added to the ident `:people/all`
'[({:people/all [:person/name :person/age]}
   {:offset 20 :limit 10})]

These can be quite hard for human to follow if some child join also have parameters themselves. Fortunately, as you are using om.next / fulcro to build these queries from smaller ones from components, that would be no problem at all.

Walkable can work with both new and legacy syntax for params. However the new one is only supported as of fulcro 2.2.1. If you're using om.next or older fulcro, you must use the legacy syntax otherwise your client-side app will crash.

Practise

It's recommended to get acquainted with the query language by playing with the parser. Give some arbitrary query to com.wsscode.pathom.core/query->ast and see the output. For example:

Code
Output
(require [com.wsscode.pathom.core :refer [query->ast]])

(query->ast [:foo :bar {:yup [:that]}])
{:type :root,
 :children [{:type :prop, :dispatch-key :foo, :key :foo}
            {:type :prop, :dispatch-key :bar, :key :bar}
            {:type :join, :dispatch-key :yup, :key :yup, :query [:that],
             :children [{:type :prop, :dispatch-key :that, :key :that}]}]}

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