Basic support for TOP.

Author: RyanLamansky

Example/Program.cs

@@ -24,7 +24,7 @@
 // The simulation state is preserved across EF DbContexts.
 using (var context = new SimulatedContext(simulation))
 {
-    var receivedValue = context.ExampleRecord.Select(x => x.Id).AsEnumerable();
+    var receivedValue = context.ExampleRecord.Select(x => x.Id);
 
     Console.Write(receivedValue.FirstOrDefault()); // Will write "1", as we stored earlier.
 }

README.md

@@ -31,7 +31,7 @@ using (var context = new SimulatedContext(simulation))
 // The simulation state is preserved across EF DbContexts.
 using (var context = new SimulatedContext(simulation))
 {
-    var receivedValue = context.ExampleRecord.Select(x => x.Id).AsEnumerable();
+    var receivedValue = context.ExampleRecord.Select(x => x.Id);
 
     Console.Write(receivedValue.FirstOrDefault()); // Will write "1", as we stored earlier.
 }

SqlServerSimulator.Tests.EFCore/EFCoreBasics.cs

@@ -103,4 +103,50 @@ public void RoundTrip()
             Assert.AreEqual(storedValue, receivedValue.FirstOrDefault());
         }
     }
+
+    [TestMethod]
+    public void SeparateInserts()
+    {
+        var simulation = CreateDefaultSimulation();
+        int[] storedValues = [2, 3];
+
+        using (var context = new TestDbContext(simulation))
+        {
+            foreach (var value in storedValues)
+            {
+                var row = new TestRow { Id = value };
+                _ = context.Rows.Add(row);
+                _ = context.SaveChanges();
+            }
+        }
+
+        using (var context = new TestDbContext(simulation))
+        {
+            CollectionAssert.AreEquivalent(storedValues, context.Rows.Select(x => x.Id).ToArray());
+        }
+    }
+
+    [TestMethod]
+    public void FirstOrDefault()
+    {
+        var simulation = CreateDefaultSimulation();
+        int[] storedValues = [4, 5];
+
+        using (var context = new TestDbContext(simulation))
+        {
+            foreach (var value in storedValues)
+            {
+                var row = new TestRow { Id = value };
+                _ = context.Rows.Add(row);
+                _ = context.SaveChanges();
+            }
+        }
+
+        using (var context = new TestDbContext(simulation))
+        {
+            var receivedValue = context.Rows.Select(x => x.Id);
+            // Without an OrderBy, we can't guarantee which of the two possibilities is returned.
+            CollectionAssert.Contains(storedValues, receivedValue.FirstOrDefault());
+        }
+    }
 }

SqlServerSimulator.Tests/SelectTests.cs

@@ -196,4 +196,18 @@ public void DerivedTable(string commandText, string name, object value)
 
         Assert.AreEqual(value, result);
     }
+
+    [TestMethod]
+    public void TopParenthesizedConstantUnsorted()
+    {
+        CollectionAssert.AreEquivalent([1], [.. new Simulation()
+            .ExecuteReader("select top (1) 1")
+            .EnumerateRecords()
+            .Select(reader => (int)reader[0])], EqualityComparer<int>.Default);
+
+        CollectionAssert.AreEquivalent([], [.. new Simulation()
+            .ExecuteReader("select top (0) 1")
+            .EnumerateRecords()
+            .Select(reader => (int)reader[0])], EqualityComparer<int>.Default);
+    }
 }

SqlServerSimulator/Parser/Selection.cs

@@ -12,8 +12,34 @@ public static Selection Parse(Simulation simulation, IEnumerator<Token> tokens,
     {
         token = tokens.RequireNext();
 
+        int? topCount = null;
+
+        if (token is ReservedKeyword { Keyword: Keyword.Top })
+        {
+            // SQL Server doesn't require outer parentheses.
+            // When Expression.Parse supports them, the checks for them here should be removed.
+            token = tokens.RequireNext<OpenParentheses>();
+            token = tokens.RequireNext();
+
+            var resolvedExpression = Expression.Parse(simulation, tokens, ref token, getVariableValue).Run(name => throw SimulatedSqlException.ColumnReferenceNotAllowed(name));
+            topCount = resolvedExpression is int unboxed ? unboxed : throw SimulatedSqlException.TopFetchRequiresInteger();
+
+            if (token is not null and not CloseParentheses)
+                throw SimulatedSqlException.SyntaxErrorNear(token);
+
+            token = tokens.RequireNext();
+        }
+
         List<Expression> expressions = [];
 
+        IEnumerable<object?[]> ApplyClauses(IEnumerable<object?[]> records)
+        {
+            if (topCount is not null)
+                records = records.Take(topCount.GetValueOrDefault());
+
+            return records;
+        }
+
         do
         {
             expressions.Add(Expression.Parse(simulation, tokens, ref token, getVariableValue));
@@ -32,7 +58,7 @@ public static Selection Parse(Simulation simulation, IEnumerator<Token> tokens,
                 case null: // "Select" with no "From".
                     return new(new(
                         expressions,
-                        [[.. expressions.Select(x => x.Run(column => throw SimulatedSqlException.InvalidColumnName(column)))]]
+                        ApplyClauses([[.. expressions.Select(x => x.Run(column => throw SimulatedSqlException.InvalidColumnName(column)))]])
                         ));
 
                 case ReservedKeyword expectFrom:
@@ -60,11 +86,11 @@ public static Selection Parse(Simulation simulation, IEnumerator<Token> tokens,
 
                             return new(new(
                                 expressions,
-                                table.Rows.Select<object?[], object?[]>(row => [..expressions.Select(x => x.Run(columnName =>
+                                ApplyClauses(table.Rows.Select<object?[], object?[]>(row => [..expressions.Select(x => x.Run(columnName =>
                                 {
                                     var columnIndex = table.Columns.FindIndex(column => Collation.Default.Equals(column.Name, columnName.Last()));
                                     return columnIndex == -1 ? throw SimulatedSqlException.InvalidColumnName(columnName) : row[columnIndex];
-                                }))])
+                                }))]))
                                 ));
 
                         case OpenParentheses:
@@ -86,11 +112,11 @@ public static Selection Parse(Simulation simulation, IEnumerator<Token> tokens,
 
                                 return new(new(
                                     expressions,
-                                    derived.Select<object?[], object?[]>(row => [..expressions.Select(x => x.Run(columnName =>
+                                    ApplyClauses(derived.Select<object?[], object?[]>(row => [..expressions.Select(x => x.Run(columnName =>
                                     {
                                         var columnIndex = Array.FindIndex(derived.columnNames, name => Collation.Default.Equals(name, columnName.Last()));
                                         return columnIndex == -1 ? throw SimulatedSqlException.InvalidColumnName(columnName) : row[columnIndex];
-                                    }))])
+                                    }))]))
                                     ));
                             }
                     }

SqlServerSimulator/SimulatedSqlException.cs

@@ -92,6 +92,14 @@ private SimulatedSqlException(string? message, params ReadOnlySpan<SimulatedSqlE
     /// <returns>The exception.</returns>
     internal static SimulatedSqlException CannotFindDataType(string name, int index) => new($"Column, parameter, or variable #{index}: Cannot find data type {name}.", 2715, 16, 6);
 
+    /// <summary>
+    /// Mimics the SqlException that occurs then when a TOP/OFFSET/FETCH clause has an inappropriate column reference.
+    /// </summary>
+    /// <param name="name">The name of the column.</param>
+    /// <returns>The exception.</returns>
+    internal static SimulatedSqlException ColumnReferenceNotAllowed(IEnumerable<string> name)
+        => new($"The reference to column \"{string.Join('.', name)}\" is not allowed in an argument to a TOP, OFFSET, or FETCH clause. Only references to columns at an outer scope or standalone expressions and subqueries are allowed here.", 4115, 15, 1);
+
     internal static SimulatedSqlException InvalidColumnName(string name) => new($"Invalid column name '{name}'.", 207, 16, 1);
 
     internal static SimulatedSqlException InvalidColumnName(IEnumerable<string> name) => InvalidColumnName(string.Join('.', name));
@@ -104,5 +112,11 @@ private SimulatedSqlException(string? message, params ReadOnlySpan<SimulatedSqlE
 
     internal static SimulatedSqlException ThereIsAlreadyAnObject(string name) => new($"There is already an object named '{name}' in the database.", 2714, 16, 6);
 
+    /// <summary>
+    /// Mimics the SqlException that occurs then when a TOP or FETCH clause returns something other than an integer.
+    /// </summary>
+    /// <returns>The exception.</returns>
+    internal static SimulatedSqlException TopFetchRequiresInteger() => new("The number of rows provided for a TOP or FETCH clauses row count parameter must be an integer.", 1060, 15, 1);
+
     internal static SimulatedSqlException UnrecognizedBuiltInFunction(string name) => new($"'{name}' is not a recognized built-in function name.", 195, 15, 10);
 }