Add pagination support to monger.query DSL

An example:

(with-collection coll
                  (find {})
                  (paginate :page 1 :per-page 3)
                  (sort { :title 1 }))

src/monger/pagination.clj

@@ -0,0 +1,6 @@
+(ns monger.pagination)
+
+(defn offset-for
+  [^long page ^long per-page]
+  (* per-page
+     (- (max page 1) 1)))

src/monger/query.clj

@@ -80,6 +80,10 @@
   [m]
   (merge m { :snapshot true }))
 
+(defn paginate
+  [m & { :keys [page per-page] :or { page 1 per-page 10 } }]
+  (merge m { :limit per-page :skip (monger.pagination/offset-for page per-page) }))
+
 (defmacro with-collection
   [^String coll & body]
   `(binding [*query-collection* (if (string? ~coll)

test/monger/test/pagination.clj

@@ -0,0 +1,14 @@
+(ns monger.test.pagination
+  (:use [clojure.test]
+        [monger.pagination]))
+
+(deftest test-pagination-offset
+  (are [a b] (= a b)
+       0 (offset-for 1 20)
+       0 (offset-for 1 30)
+       10 (offset-for 2 10)
+       13 (offset-for 2 13)
+       20 (offset-for 3 10)
+       22 (offset-for 3 11)
+       21 (offset-for 4 7)
+       39 (offset-for 4 13)))

test/monger/test/querying.clj

@@ -178,3 +178,34 @@
         result2 (mgcol/find-one-as-map coll { :counter { "$ne" 32 } })]
     (is (= doc2 result1))
     (is (= doc1 result2))))
+
+;;
+;; monger.query DSL features
+;;
+
+;; pagination
+(deftest query-using-pagination-dsl
+  (let [coll "docs"
+        doc1 { :_id (ObjectId.) :title "Clojure" :tags ["functional" "homoiconic" "syntax-oriented" "dsls" "concurrency features" "jvm"] }
+        doc2 { :_id (ObjectId.) :title "Java"    :tags ["object-oriented" "jvm"] }
+        doc3 { :_id (ObjectId.) :title "Scala"   :tags ["functional" "object-oriented" "dsls" "concurrency features" "jvm"] }
+        doc4 { :_id (ObjectId.) :title "Ruby"    :tags ["dynamic" "object-oriented" "dsls" "jvm"] }
+        doc5 { :_id (ObjectId.) :title "Groovy"  :tags ["dynamic" "object-oriented" "dsls" "jvm"] }
+        doc6 { :_id (ObjectId.) :title "OCaml"   :tags ["functional" "static" "dsls"] }
+        doc7 { :_id (ObjectId.) :title "Haskell" :tags ["functional" "static" "dsls" "concurrency features"] }
+        -    (mgcol/insert-batch coll [doc1 doc2 doc3 doc4 doc5 doc6 doc7])
+        result1 (with-collection coll
+                  (find {})
+                  (paginate :page 1 :per-page 3)
+                  (sort { :title 1 }))
+        result2 (with-collection coll
+                  (find {})
+                  (paginate :page 2 :per-page 3)
+                  (sort { :title 1 }))
+        result3 (with-collection coll
+                  (find {})
+                  (paginate :page 3 :per-page 3)
+                  (sort { :title 1 }))]
+    (is (= [doc1 doc5 doc7] result1))
+    (is (= [doc2 doc6 doc4] result2))
+    (is (= [doc3] result3))))