From 84012c12c52aa574f8b464c7ee7c603b9dc3bb61 Mon Sep 17 00:00:00 2001
From: manavggupta <manav1878.be22@chitkara.edu.in>
Date: Sun, 22 Feb 2026 23:15:44 +0530
Subject: [PATCH] Fix: Validate join columns before performing join to prevent
 unintended cartesian product (#118)

---
 src/DataFrame/Operations/Join.hs | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/src/DataFrame/Operations/Join.hs b/src/DataFrame/Operations/Join.hs
index 06a66971..f62aa62e 100644
--- a/src/DataFrame/Operations/Join.hs
+++ b/src/DataFrame/Operations/Join.hs
@@ -28,6 +28,21 @@ data JoinType
     | RIGHT
     | FULL_OUTER
 
+    validateJoinColumns :: [T.Text] -> DataFrame -> DataFrame -> ()
+validateJoinColumns cs df1 df2 =
+    let
+        df1Cols = D.columnNames df1
+        df2Cols = D.columnNames df2
+
+        missingInDf1 = filter (`notElem` df1Cols) cs
+        missingInDf2 = filter (`notElem` df2Cols) cs
+
+        missing = missingInDf1 ++ missingInDf2
+     in
+        if not (null missing)
+            then error $ "Column not found: " <> show missing
+            else ()
+
 {- | Join two dataframes using SQL join semantics.
 
 Only inner join is implemented for now.
@@ -66,6 +81,7 @@ ghci> D.innerJoin ["key"] df other
 innerJoin :: [T.Text] -> DataFrame -> DataFrame -> DataFrame
 innerJoin cs right left =
     let
+        _ = validateJoinColumns cs right left
         -- Prepare Keys for the Right DataFrame
         rightIndicesToGroup =
             [c | (k, c) <- M.toList (D.columnIndices right), k `elem` cs]
@@ -170,6 +186,7 @@ leftJoin ::
     [T.Text] -> DataFrame -> DataFrame -> DataFrame
 leftJoin cs right left =
     let
+        _ = validateJoinColumns cs right left
         leftIndicesToGroup = M.elems $ M.filterWithKey (\k _ -> k `elem` cs) (D.columnIndices left)
         leftRowRepresentations = D.computeRowHashes leftIndicesToGroup left
         rightIndicesToGroup = M.elems $ M.filterWithKey (\k _ -> k `elem` cs) (D.columnIndices right)
@@ -250,6 +267,7 @@ fullOuterJoin ::
     [T.Text] -> DataFrame -> DataFrame -> DataFrame
 fullOuterJoin cs right left =
     let
+        _ = validateJoinColumns cs right left
         leftIndicesToGroup = M.elems $ M.filterWithKey (\k _ -> k `elem` cs) (D.columnIndices left)
         leftRowRepresentations = D.computeRowHashes leftIndicesToGroup left
         leftKeyCountsAndIndices =