Fix #27 by casting literal strings to varchar in projection lists only

src/EntityFramework6.Npgsql/EntityFramework6.Npgsql.csproj

@@ -69,6 +69,7 @@
     <Compile Include="NpgsqlServices.cs" />
     <Compile Include="NpgsqlProviderManifest.cs" />
     <Compile Include="NpgsqlTextFunctions.cs" />
+    <Compile Include="NpgsqlTypeFunctions.cs" />
     <Compile Include="NpgsqlWeightLabel.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Spatial\PostgisDataReader.cs" />

src/EntityFramework6.Npgsql/NpgsqlProviderManifest.cs

@@ -351,15 +351,15 @@ public override string EscapeLikeArgument([NotNull] string argument)
         public override bool SupportsInExpression() => true;
 
         public override ReadOnlyCollection<EdmFunction> GetStoreFunctions()
-            => typeof(NpgsqlTextFunctions).GetTypeInfo()
-                .GetMethods(BindingFlags.Public | BindingFlags.Static)
+            => new[] { typeof(NpgsqlTextFunctions).GetTypeInfo(), typeof(NpgsqlTypeFunctions) }
+                .SelectMany(x => x.GetMethods(BindingFlags.Public | BindingFlags.Static))
                 .Select(x => new { Method = x, DbFunction = x.GetCustomAttribute<DbFunctionAttribute>() })
                 .Where(x => x.DbFunction != null)
-                .Select(x => CreateFullTextEdmFunction(x.Method, x.DbFunction))
+                .Select(x => CreateComposableEdmFunction(x.Method, x.DbFunction))
                 .ToList()
                 .AsReadOnly();
 
-        static EdmFunction CreateFullTextEdmFunction([NotNull] MethodInfo method, [NotNull] DbFunctionAttribute dbFunctionInfo)
+        static EdmFunction CreateComposableEdmFunction([NotNull] MethodInfo method, [NotNull] DbFunctionAttribute dbFunctionInfo)
         {
             if (method == null)
                 throw new ArgumentNullException(nameof(method));

src/EntityFramework6.Npgsql/NpgsqlTypeFunctions.cs

@@ -0,0 +1,20 @@
+using System;
+using System.Data.Entity;
+
+namespace Npgsql
+{
+    /// <summary>
+    /// Use this class in LINQ queries to emit type manipulation SQL fragments.
+    /// </summary>
+    public static class NpgsqlTypeFunctions
+    {
+        /// <summary>
+        /// Emits an explicit cast for unknown types sent as strings to their correct postgresql type.
+        /// </summary>
+        [DbFunction("Npgsql", "cast")]
+        public static string Cast(string unknownTypeValue, string postgresTypeName)
+        {
+            throw new NotSupportedException();
+        }
+    }
+}

src/EntityFramework6.Npgsql/SqlGenerators/SqlBaseGenerator.cs

@@ -322,7 +322,14 @@ PendingProjectsNode VisitInputWithBinding(DbExpression expression, string bindin
                 for (var i = 0; i < rowType.Properties.Count && i < projection.Arguments.Count; ++i)
                 {
                     var prop = rowType.Properties[i];
-                    input.Projection.Arguments.Add(new ColumnExpression(projection.Arguments[i].Accept(this), prop.Name, prop.TypeUsage));
+                    var argument = projection.Arguments[i].Accept(this);
+                    var constantArgument = projection.Arguments[i] as DbConstantExpression;
+                    if (constantArgument != null && constantArgument.Value is string)
+                    {
+                        argument = new CastExpression(argument, "varchar");
+                    }
+
+                    input.Projection.Arguments.Add(new ColumnExpression(argument, prop.Name, prop.TypeUsage));
                 }
 
                 if (enterScope) LeaveExpression(child);
@@ -1150,6 +1157,17 @@ VisitedExpression VisitFunction(EdmFunction function, IList<DbExpression> args,
                 {
                     return VisitMatchRegex(function, args, resultType);
                 }
+                else if (functionName == "cast")
+                {
+                    if (args.Count != 2)
+                        throw new ArgumentException("Invalid number of arguments. Expected 2.", "args");
+
+                    var typeNameExpression = args[1] as DbConstantExpression;
+                    if (typeNameExpression == null)
+                        throw new NotSupportedException("cast type name argument must be a constant expression.");
+
+                    return new CastExpression(args[0].Accept(this), typeNameExpression.Value.ToString());
+                }
             }
 
             var customFuncCall = new FunctionExpression(

test/EntityFramework6.Npgsql.Tests/EntityFrameworkBasicTests.cs

@@ -650,5 +650,74 @@ public void TestScalarValuedStoredFunctions_with_null_StoreFunctionName()
                 Assert.That(echo, Is.EqualTo(1337));
             }
         }
+
+        [Test]
+        public void TestCastFunction()
+        {
+            using (var context = new BloggingContext(ConnectionString))
+            {
+                context.Database.Log = Console.Out.WriteLine;
+
+                var varbitVal = "10011";
+
+                var blog = new Blog
+                {
+                    Name = "_",
+                    Posts = new List<Post>
+                    {
+                        new Post
+                        {
+                            Content = "Some post content",
+                            Rating = 1,
+                            Title = "Some post Title",
+                            VarbitColumn = varbitVal
+                        }
+                    }
+                };
+                context.Blogs.Add(blog);
+                context.SaveChanges();
+
+                Assert.IsTrue(
+                    context.Posts.Select(
+                        p => NpgsqlTypeFunctions.Cast(p.VarbitColumn, "varbit") == varbitVal).First());
+
+                Assert.IsTrue(
+                    context.Posts.Select(
+                        p => NpgsqlTypeFunctions.Cast(p.VarbitColumn, "varbit") == "10011").First());
+            }
+        }
+
+        [Test]
+        public void Test_issue_27_select_ef_generated_literals_from_inner_select()
+        {
+            using (var context = new BloggingContext(ConnectionString))
+            {
+                context.Database.Log = Console.Out.WriteLine;
+
+                var blog = new Blog { Name = "Hello" };
+                context.Users.Add(new Administrator { Blogs = new List<Blog> { blog } });
+                context.Users.Add(new Editor());
+                context.SaveChanges();
+
+                var administrator = context.Users
+                    .Where(x => x is Administrator) // Removing this changes the query to using a UNION which doesn't fail.
+                    .Select(
+                        x => new
+                        {
+                           // causes entity framework to emit a literal discriminator
+                           Computed = x is Administrator
+                                ? "I administrate"
+                                : x is Editor
+                                    ? "I edit"
+                                    : "Unknown",
+                           // causes an inner select to be emitted thus showing the issue
+                           HasBlog = x.Blogs.Any()
+                        })
+                    .First();
+
+                Assert.That(administrator.Computed, Is.EqualTo("I administrate"));
+                Assert.That(administrator.HasBlog, Is.True);
+            }
+        }
     }
 }

test/EntityFramework6.Npgsql.Tests/Support/EntityFrameworkTestBase.cs

@@ -104,6 +104,20 @@ public class NoColumnsEntity
         public int Id { get; set; }
     }
 
+    [Table("Users")]
+    public abstract class User
+    {
+        public int Id { get; set; }
+
+        public IList<Blog> Blogs { get; set; }
+    }
+
+    [Table("Editors")]
+    public class Editor : User { }
+
+    [Table("Administrators")]
+    public class Administrator : User { }
+
     public class BloggingContext : DbContext
     {
         public BloggingContext(string connection)
@@ -114,6 +128,9 @@ public BloggingContext(string connection)
         public DbSet<Blog> Blogs { get; set; }
         public DbSet<Post> Posts { get; set; }
         public DbSet<NoColumnsEntity> NoColumnsEntities { get; set; }
+        public DbSet<User> Users { get; set; }
+        public DbSet<Editor> Editors { get; set; }
+        public DbSet<Administrator> Administrators { get; set; }
 
         [DbFunction("BloggingContext", "ClrStoredAddFunction")]
         public static int StoredAddFunction(int val1, int val2)
@@ -135,6 +152,9 @@ private static DbCompiledModel CreateModel(NpgsqlConnection connection)
             dbModelBuilder.Entity<Blog>();
             dbModelBuilder.Entity<Post>();
             dbModelBuilder.Entity<NoColumnsEntity>();
+            dbModelBuilder.Entity<User>();
+            dbModelBuilder.Entity<Editor>();
+            dbModelBuilder.Entity<Administrator>();
 
             // Import function
             var dbModel = dbModelBuilder.Build(connection);