Add default query term

The default function lets you provide a default value/function when a
field or value is null or missing.

Fixes #29, fixes #99

CHANGELOG.md

@@ -7,6 +7,7 @@ All notable changes to this project will be documented in this file. This change
 - clojure.tools.logging support. [#72](https://github.com/apa512/clj-rethinkdb/pull/72)
 - `fn` macro to ClojureScript `rethinkdb.query` namespace. [#64](https://github.com/apa512/clj-rethinkdb/issues/64)
 - `rethinkdb.query/order-by` function to ClojureScript. [#65](https://github.com/apa512/clj-rethinkdb/issues/65)
+- `rethinkdb.query/default` function, for supplying a default value/function for missing values.
 
 ### Changed
 - Update dependency to Clojure 1.7. [#59](https://github.com/apa512/clj-rethinkdb/pull/59)

src/rethinkdb/query.cljc

@@ -781,6 +781,15 @@
   ([] (term :ERROR []))
   ([s] (term :ERROR [s])))
 
+(defn default
+  "Provide a default value in case of non-existence errors. default evaluates its
+  first argument (the value it’s chained to). If that argument returns null or a
+  non-existence error is thrown in evaluation, then default returns its second argument.
+  The second argument is usually a default value, but it can be a RethinkDB function that
+  returns a value."
+  [any default]
+  (term :DEFAULT [any default]))
+
 (defn coerce-to
   "Convert a value of one type into another."
   [x s]

test/rethinkdb/core_test.clj

@@ -113,7 +113,14 @@
       (is (= (r/run (-> (r/db test-db)
                         (r/table test-table)
                         (r/filter (r/fn [row]
-                                    (r/eq (r/get-field row :name) "Pikachu")))) conn) [(first pokemons)])))))
+                                    (r/eq (r/get-field row :name) "Pikachu")))) conn) [(first pokemons)]))
+      (is (= 1 (r/run (r/get-field {:a 1} :a) conn))))
+
+    (testing "default values"
+      (is (= "not found" (r/run (-> (r/get-field {:a 1} :b) (r/default "not found")) conn)))
+      (is (= "not found" (r/run (-> (r/max [nil]) (r/default "not found")) conn)))
+      (is (= "Cannot take the average of an empty stream.  (If you passed `avg` a field name, it may be that no elements of the stream had that field.)"
+             (r/run (-> (r/avg [nil]) (r/default (r/fn [row] row))) conn))))))
 
 (deftest db-in-connection
   (testing "run a query with an implicit database"