fix: resolve self-join issue

sqlframe/base/dataframe.py

@@ -866,6 +866,25 @@ def join(
         if on is None:
             logger.warning("Got no value for on. This appears to change the join to a cross join.")
             how = "cross"
+        on_cols = self._ensure_list_of_columns(on)
+        # If we are joining in another dataframe that comes from the same branch, we need to treat the other dataframe
+        # as a new branch so we properly differentiate the two dataframes in the final SQL output.
+        if self.branch_id == other_df.branch_id:
+            new_branch_id = self.session._random_branch_id
+            replacement_mapping = {
+                exp.to_identifier(other_df.branch_id): exp.to_identifier(new_branch_id)
+            }
+            other_df.branch_id = new_branch_id
+            other_df.expression = other_df.expression.transform(
+                replace_id_value, replacement_mapping
+            ).assert_is(exp.Select)
+            for col in on_cols:
+                # We only want to update one side of the join if EQ so we find all EQs and update the right side
+                for eq in col.expression.find_all(exp.EQ):
+                    eq.set(
+                        "expression", eq.expression.transform(replace_id_value, replacement_mapping)
+                    )
+
         other_df = other_df._convert_leaf_to_cte()
         join_expression = self._add_ctes_to_expression(self.expression, other_df.expression.ctes)
         # We will determine actual "join on" expression later so we don't provide it at first
@@ -874,7 +893,7 @@ def join(
         )
         self_columns = self._get_outer_select_columns(join_expression)
         other_columns = self._get_outer_select_columns(other_df.expression)
-        join_columns = self._ensure_and_normalize_cols(on)
+        join_columns = self._ensure_and_normalize_cols(on_cols)
         # Determines the join clause and select columns to be used passed on what type of columns were provided for
         # the join. The columns returned changes based on how the on expression is provided.
         if how != "cross":

tests/integration/test_int_dataframe.py

@@ -2247,3 +2247,28 @@ def test_chaining_joins_with_selects(
     )
 
     compare_frames(df, dfs, compare_schema=False)
+
+
+# https://github.com/eakmanrq/sqlframe/issues/210
+def test_self_join(
+    pyspark_employee: PySparkDataFrame,
+    get_df: t.Callable[[str], _BaseDataFrame],
+    compare_frames: t.Callable,
+):
+    df_filtered = pyspark_employee.where(F.col("age") > 40)
+    df_joined = pyspark_employee.join(
+        df_filtered,
+        pyspark_employee["employee_id"] == df_filtered["employee_id"],
+        how="inner",
+    )
+
+    employee = get_df("employee")
+
+    dfs_filtered = employee.where(SF.col("age") > 40)
+    dfs_joined = employee.join(
+        dfs_filtered,
+        employee["employee_id"] == dfs_filtered["employee_id"],
+        how="inner",
+    )
+
+    compare_frames(df_joined, dfs_joined, compare_schema=False)