Implement get-else.

src/datomish/query/functions.cljc

@@ -6,7 +6,12 @@
   (:require
      [honeysql.format :as fmt]
      [datomish.query.cc :as cc]
-     [datomish.query.source :as source]
+     [datomish.schema :as schema]
+     [datomish.sqlite-schema :refer [->tag ->SQLite]]
+     [datomish.query.source
+      :as source
+      :refer [attribute-in-source
+              constant-in-source]]
      [datomish.util :as util #?(:cljs :refer-macros :clj :refer) [raise raise-str cond-let]]
      [datascript.parser :as dp
       #?@(:cljs
@@ -136,9 +141,95 @@
 
     (cc/augment-cc cc from bindings wheres)))
 
+;; get-else is how Datalog handles optional attributes.
+;;
+;; It consists of:
+;; * A bound entity
+;; * A cardinality-one attribute
+;; * A var to bind the value
+;; * A default value.
+;;
+;; We model this as:
+;; * A check against known bindings for the entity.
+;; * A check against the schema for cardinality-one.
+;; * Generating a COALESCE expression with a query inside the projection itself.
+;;
+;; Note that this will be messy for queries like:
+;;
+;;   [:find ?page ?title :in $
+;;    :where [?page :page/url _]
+;;           [(get-else ?page :page/title "<empty>") ?title]
+;;           [_ :foo/quoted ?title]]
+;;
+;; or
+;;           [(some-function ?title)]
+;;
+;; -- we aren't really establishing a binding, so the subquery will be
+;; repeated. But this will do for now.
+(defn apply-get-else-clause [cc function]
+  (let [{:keys [source bindings external-bindings]} cc
+        schema (:schema source)
+
+        {:keys [args binding]} function
+        [src e a default-val] args]
+
+    (when-not (instance? BindScalar binding)
+      (raise-str "Expected scalar binding."))
+    (when-not (instance? Variable (:variable binding))
+      (raise-str "Expected variable binding."))
+    (when-not (instance? Constant a)
+      (raise-str "Expected constant attribute."))
+    (when-not (instance? Constant default-val)
+      (raise-str "Expected constant default value."))
+    (when-not (and (instance? SrcVar src)
+                   (= "$" (name (:symbol src))))
+      (raise "Non-default sources not supported." {:arg src}))
+
+    (let [a (attribute-in-source source (:value a))
+          a-type (get-in (:schema schema) [a :db/valueType])
+          a-tag (->tag a-type)
+
+          default-val (:value default-val)
+          var (:variable binding)]
+
+      ;; Schema check.
+      (when-not (and (integer? a)
+                     (not (datomish.schema/multival? schema a)))
+        (raise-str "Attribute " a " is not cardinality-one."))
+
+      ;; TODO: type-check the default value.
+
+      (condp instance? e
+        Variable
+        (let [e (:symbol e)
+              e-binding (cc/binding-for-symbol-or-throw cc e)]
+
+          (let [[table _] (source/source->from source a)  ; We don't need to alias: single pattern.
+                ;; These :limit values shouldn't be needed, but sqlite will
+                ;; appreciate them.
+                ;; Note that we don't extract type tags here: the attribute
+                ;; must be known!
+                subquery {:select
+                          [(sql/call
+                             :coalesce
+                             {:select [:v]
+                              :from [table]
+                              :where [:and
+                                      [:= 'a a]
+                                      [:= 'e e-binding]]
+                              :limit 1}
+                             (->SQLite default-val))]
+                          :limit 1}]
+            (->
+              (assoc-in cc [:known-types (:symbol var)] a-type)
+              (util/append-in [:bindings (:symbol var)] subquery))))
+
+        (raise-str "Can't handle entity" e)))))
+
 (def sql-functions
   ;; Future: versions of this that uses snippet() or matchinfo().
-  {"fulltext" apply-fulltext-clause})
+  {"fulltext" apply-fulltext-clause
+   "get-else" apply-get-else-clause})
 
 (defn apply-sql-function
   "Either returns an application of `function` to `cc`, or nil to

src/datomish/query/source.cljc

@@ -6,6 +6,7 @@
   (:require
    [datomish.query.transforms :as transforms]
    [datomish.schema :as schema]
+   [datomish.util :as util #?(:cljs :refer-macros :clj :refer) [raise raise-str]]
    [datascript.parser
     #?@(:cljs
         [:refer [Variable Constant Placeholder]])])
@@ -68,15 +69,31 @@
   Source
 
   (source->from [source attribute]
-    (let [table
+    (let [schema (:schema source)
-          (if (and (instance? Constant attribute)
+          int->table (fn [a]
-                   ;; TODO: look in the DB schema to see if `attribute` is known to not be
+                       (if (schema/fulltext? schema a)
-                   ;; a fulltext attribute.
+                         (:fulltext-table source)
-                   true)
+                         (:table source)))
-            (:table source)
+          table
+          (cond
+            (integer? attribute)
+            (int->table attribute)
 
+            (instance? Constant attribute)
+            (let [a (:value attribute)
+                  id (if (keyword? a)
+                       (attribute-in-source source a)
+                       a)]
+              (int->table id))
+
+            ;; TODO: perhaps we know an external binding already?
+            (or (instance? Variable attribute)
+                (instance? Placeholder attribute))
             ;; It's variable. We must act as if it could be a fulltext datom.
-            (:fulltext-view source))]
+            (:fulltext-view source)
+
+            true
+            (raise "Unknown source->from attribute " attribute {:attribute attribute}))]
       [table ((:table-alias source) table)]))
 
   (source->non-fulltext-from [source]

test/datomish/test/query.cljc

@@ -494,3 +494,26 @@
                               [:= :datoms0.e :datoms1.e])}}
             :from [:preag]}
            (query/context->sql-clause context)))))
+
+(deftest-db test-get-else conn
+  (let [attrs (<? (<initialize-with-schema conn page-schema))]
+    (is (= {:select (list
+                      [:datoms0.e :page]
+                      [{:select [(sql/call
+                                   :coalesce
+                                   {:select [:v],
+                                    :from [:datoms],
+                                    :where [:and
+                                            [:= 'a 65540]
+                                            [:= 'e :datoms0.e]],
+                                    :limit 1}
+                                   "No title")],
+                        :limit 1} :title]),
+            :modifiers [:distinct],
+            :from '([:datoms datoms0]),
+            :where (list :and [:= :datoms0.a 65539])}
+           (expand '[:find ?page ?title :in $
+                     :where
+                     [?page :page/url _]
+                     [(get-else $ ?page :page/title "No title") ?title]]
+                   conn)))))