Bug fix: Fix issue where statements where not downcasted to IDataValue for case when (#816)

Author: Ulimo

src/FlowtideDotNet.Core/Compute/Columnar/ColumnarExpressionVisitor.cs

@@ -210,6 +210,11 @@ private static IDataValue GetColumnValue(in EventBatchData data, in int column,
                 elseStatement = System.Linq.Expressions.Expression.Constant(new NullValue(), typeof(IDataValue));
             }
 
+            if (elseStatement.Type != typeof(IDataValue))
+            {
+                elseStatement = System.Linq.Expressions.Expression.Convert(elseStatement, typeof(IDataValue));
+            }
+
             var expr = elseStatement;
             for (int i = ifThenExpression.Ifs.Count - 1; i >= 0; i--)
             {
@@ -227,6 +232,11 @@ private static IDataValue GetColumnValue(in EventBatchData data, in int column,
                     ifStatement = System.Linq.Expressions.Expression.Call(toBoolMethod, ifStatement);
                 }
 
+                if (thenStatement.Type != typeof(IDataValue))
+                {
+                    thenStatement = System.Linq.Expressions.Expression.Convert(thenStatement, typeof(IDataValue));
+                }
+
                 expr = System.Linq.Expressions.Expression.Condition(ifStatement, thenStatement, expr);
             }
 

tests/FlowtideDotNet.AcceptanceTests/SelectTests.cs

@@ -78,6 +78,25 @@ END AS name
             AssertCurrentDataEqual(Users.Select(x => new { Name = x.Gender == Entities.Gender.Female ? x.FirstName : x.LastName }));
         }
 
+        /// <summary>
+        /// Special case test where the case did not downcast correctly to IDataValue
+        /// </summary>
+        /// <returns></returns>
+        [Fact]
+        public async Task SelectWithCaseStructWithNullElse()
+        {
+            GenerateData();
+            await StartStream(@"
+                INSERT INTO output 
+                SELECT 
+                    CASE WHEN Active = false THEN named_struct('id', FirstName) 
+                    ELSE NULL
+                    END AS name
+                FROM users");
+            await WaitForUpdate();
+            AssertCurrentDataEqual(Users.Select(x => new { Name = x.Active == false ? new { id = x.FirstName } : null }));
+        }
+
         [Fact]
         public async Task SelectWithCaseNoElse()
         {