Pulling maps from sequences requires named keys

[flybot-nam-nguyenhoai]

Problem

When pulling from a sequence of maps, lasagna-pull requires all map keys to be specified up front. For example:

((pull/query ['?]) [{:a 1} {:a 2 :b 1}]) ;=> Exception: Wrong pattern
((pull/query [{:a '?}]) [{:a 1} {:a 2 :b 1}]) ;=> [{:a 1} {:a 2}]

Impact

It is difficult to work with lists of entities that may not follow fixed schemas, such as when working with Datomic or re-frame.

Probable cause

schema/patten-schema-of does not handle [:sequential :map] correctly. For example:

(def data-schema
  [:vector :map])

(def pattern-schema
  (schema/pattern-schema-of data-schema))

pattern-schema
;; => [:cat {:sg.flybot.pullable.schema/pattern? true}
;; =>  :map
;; =>  [:? [:fn #function[lvar?]]]
;; =>  [:? [:alt [:cat [:= :seq] [:vector {:min 1, :max 2} :int]]]]]

(malli.core/validate pattern-schema ['?]) ;=> false
(malli.core/validate pattern-schema [{} '?]) ;=> true

Suggestion

• Allow using ['?] to pull whole elements (such as entire maps) from a sequence. For example:

  ((pull/query ['?]) [{:a 1} {:a 2 :b 1}])
  ;; => [{:a 1} {:a 2 :b 1}]

[robertluo]

One purpose of pull pattern schema is to limit the visibility; if you provide a map schema, it means that the client can only see the documented parts.