diff --git a/README.markdown b/README.markdown
--- a/README.markdown
+++ b/README.markdown
@@ -279,16 +279,20 @@
 
 >   Selects only items that meet a condition.
 
->   A *condition* in a `WHERE` statement is a Boolean combination (using
+> A *condition* in a `WHERE` statement is a Boolean combination (using
 `NOT`, `AND`, `OR`, and parentheses for disambiguation) of *basic
-conditions*.  A *basic condition* is of the form `value op value`,
-where `value` may be either a fieldname or a constant.  Note that
-all constants must be enclosed in quotes.  `op` may be one of the
-following:  `=`, `>=`, `<=`, `>`, `<` and 'contains'.
+conditions*.  A *basic condition* is either of the form `value op
+value`, where `value` may be either a fieldname or a constant, or of
+the form `HAS fieldname' where `fieldname' is a string constant.  Note
+that all constants must be enclosed in quotes.  `op` may be one of the
+following: `=`, `>=`, `<=`, `>`, `<` and 'contains'.
 
 > The basic condition `arg1 contains arg2` succeeds if and only if
 `arg1` is a fieldname whose value is a list containing the value of
 `arg2`.
+
+> The basic condition `HAS fieldname' succeeds if and only if the item
+has the fieldname as a field. For example, `HAS "data"'. 
 
 Note that the order of transformations is significant.  You can get
 different results if you use `LIMIT` before or after `ORDER BY`,
diff --git a/Yst/Data.hs b/Yst/Data.hs
--- a/Yst/Data.hs
+++ b/Yst/Data.hs
@@ -77,9 +77,11 @@
 applyDataOption _ _ = error "order by and group by can be used only on lists"
 
 satisfiesCond :: FilterCond -> Node -> Bool
-satisfiesCond (And c1 c2) n = satisfiesCond c1 n && satisfiesCond c2 n
-satisfiesCond (Or  c1 c2) n = satisfiesCond c1 n || satisfiesCond c2 n
-satisfiesCond (Not c1)    n = not (satisfiesCond c1 n)
+satisfiesCond (And c1 c2)     n = satisfiesCond c1 n && satisfiesCond c2 n
+satisfiesCond (Or  c1 c2)     n = satisfiesCond c1 n || satisfiesCond c2 n
+satisfiesCond (Not c1)        n = not (satisfiesCond c1 n)
+satisfiesCond (Has s) (NMap ns) = elem s (map fst ns)
+satisfiesCond (Has _)        _  = False
 satisfiesCond (Filter test arg1 arg2) n =
   (filterTestPred test) (filterArgToNode arg1 n) (filterArgToNode arg2 n)
 
@@ -221,7 +223,7 @@
   return $ Where cond
 
 pBooleanCondition :: GenParser Char st FilterCond
-pBooleanCondition = spaces >> (pNot <|> pAnd <|> pOr <|> pInParens pBooleanCondition <|> pBasicCond)
+pBooleanCondition = spaces >> ( pHas <|> pNot <|> pAnd <|> pOr <|> pInParens pBooleanCondition <|> pBasicCond)
 
 pInParens :: GenParser Char st a -> GenParser Char st a
 pInParens innerParser = try $ do
@@ -234,6 +236,9 @@
 
 pNot :: GenParser Char st FilterCond
 pNot = try $ pString "not" >> pSpace >> liftM Not pBooleanCondition
+
+pHas :: GenParser Char st FilterCond
+pHas = try $ pString "has" >> pSpace >> liftM Has (pQuoted '"' <|> pQuoted '\'')
 
 pAnd :: GenParser Char st FilterCond
 pAnd = try $ do
diff --git a/Yst/Types.hs b/Yst/Types.hs
--- a/Yst/Types.hs
+++ b/Yst/Types.hs
@@ -27,6 +27,7 @@
 import Data.Aeson
 import qualified Data.Map as M
 import System.Locale (defaultTimeLocale)
+import Data.Scientific (coefficient, base10Exponent)
 import Control.Monad
 
 data Site = Site {
@@ -70,6 +71,7 @@
                 | And FilterCond FilterCond
                 | Or  FilterCond FilterCond
                 | Not FilterCond
+                | Has String
                 deriving (Show, Read, Eq)
 
 data FilterArg = AttrValue String
@@ -121,7 +123,10 @@
                                Success y -> return $ NList y
                                _         -> mzero
   parseJSON (Bool b) = return $ NString $ show b
-  parseJSON (Number y) = return $ NString $ show y
+  parseJSON (Number n)
+    | base10Exponent n >= 0 = return $ NString $ show $
+                                  coefficient n * (10 ^ base10Exponent n)
+    | otherwise             = return $ NString $ show n
   parseJSON _ = return $ NNil
 
 handleMerges :: H.HashMap T.Text Value -> H.HashMap T.Text Value
diff --git a/changelog b/changelog
--- a/changelog
+++ b/changelog
@@ -1,3 +1,9 @@
+yst 0.4.1 (released 30 May 2014)
+
+  * Fix rendering of integers in YAML when compiled against
+    recent versions of aeson.
+  * Added HAS to WHERE condition in queries (Ohad Kammar).
+
 yst 0.4.0.1 (released 15 Sep 2013)
 
   * Removed unneeded build-depends.
diff --git a/yst.cabal b/yst.cabal
--- a/yst.cabal
+++ b/yst.cabal
@@ -1,5 +1,5 @@
 name:                yst
-version:             0.4.0.1
+version:             0.4.1
 Tested-With:         GHC == 7.4.1
 Cabal-version:       >= 1.8
 build-type:          Simple
@@ -56,7 +56,8 @@
                      Yst.Render, Yst.Build, Yst.CSV, Yst.Sqlite3
   build-depends:     base >=3 && < 5, unordered-containers,
                      HStringTemplate >= 0.6.1 && < 0.6.9 || > 0.6.11 && < 0.8,
-                     yaml, csv, aeson, text,
+                     yaml, csv, aeson >= 0.7 && < 0.8, text,
+                     scientific >= 0.2 && < 0.4,
                      filepath, containers, directory, time,
                      old-locale, old-time, parsec, xhtml,
                      pandoc >= 1.10 && < 1.13,