From 8488dae0a7d4340eeb6bd730305ff50f7dc0093f Mon Sep 17 00:00:00 2001
From: rbock <rbock@eudoxos.de>
Date: Sun, 11 Jan 2015 17:10:36 +0100
Subject: [PATCH] Added a few compile-time checks for unions

---
 include/sqlpp11/union.h | 54 ++++++++++++++++++++++++++++++++++-------
 tests/UnionTest.cpp     |  8 +++---
 2 files changed, 49 insertions(+), 13 deletions(-)

diff --git a/include/sqlpp11/union.h b/include/sqlpp11/union.h
index 4e65f95e..615321dc 100644
--- a/include/sqlpp11/union.h
+++ b/include/sqlpp11/union.h
@@ -37,6 +37,43 @@
 
 namespace sqlpp
 {
+	template<typename Statement, typename Enable = void>
+		struct has_result_row_impl
+		{
+			using type = std::false_type;
+		};
+
+	template<typename Statement>
+		struct has_result_row_impl<Statement, 
+		typename std::enable_if<
+			not wrong_t<typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>>::value, 
+		void>::type>
+		{
+			using type = std::true_type;
+		};
+
+	template<typename Statement>
+		using has_result_row_t = typename has_result_row_impl<Statement>::type;
+
+	template<typename Statement, typename Enable = void>
+		struct get_result_row_impl
+		{
+			using type = void;
+		};
+
+	template<typename Statement>
+		struct get_result_row_impl<Statement, 
+		typename std::enable_if<
+			not wrong_t<typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>>::value, 
+		void>::type>
+		{
+			using type = typename Statement::template _result_methods_t<Statement>::template _result_row_t<void>;
+		};
+
+	template<typename Statement>
+		using get_result_row_t = typename get_result_row_impl<Statement>::type;
+
+
 	struct no_union_t;
 
 	using blank_union_t = statement_t<void,
@@ -172,11 +209,9 @@ namespace sqlpp
 					-> _new_statement_t<_check<Rhs>, union_t<void, distinct_t, derived_statement_t<Policies>, Rhs>>
 					{
 						static_assert(is_statement_t<Rhs>::value, "argument of union call has to be a statement");
-						// Being a select might be determined by the return type?
-						//FIXME static_assert(is_select this and Rhs, "at least one argument is not an expression in union_()");
-						//FIXME static_assert(same return type, "at least one argument is not an expression in union_()");
-						//FIXME static_assert(consistent/runnable this and Rhs, "at least one expression argument required in union_()");
-						//FIXME static_assert(no order_y in this and Rhs, "at least one expression argument required in union_()");
+						static_assert(has_result_row_t<Rhs>::value, "argument of a union has to be a (complete) select statement");
+						static_assert(has_result_row_t<derived_statement_t<Policies>>::value, "left hand side argument of a union has to be a (complete) select statement");
+						static_assert(std::is_same<get_result_row_t<derived_statement_t<Policies>>, get_result_row_t<Rhs>>::value, "both select statements in a union have to have the same result columns (type and name)");
 
 						return _union_impl<void, distinct_t>(_check<derived_statement_t<Policies>, Rhs>{}, rhs);
 					}
@@ -185,10 +220,11 @@ namespace sqlpp
 					auto union_all(Rhs rhs) const
 					-> _new_statement_t<_check<Rhs>, union_t<void, all_t, derived_statement_t<Policies>, Rhs>>
 					{
-						//FIXME static_assert(is_select this and Rhs, "at least one argument is not an expression in union_()");
-						//FIXME static_assert(same return type, "at least one argument is not an expression in union_()");
-						//FIXME static_assert(consistent/runnable this and Rhs, "at least one expression argument required in union_()");
-						//FIXME static_assert(no order_y in this and Rhs, "at least one expression argument required in union_()");
+						static_assert(is_statement_t<Rhs>::value, "argument of union call has to be a statement");
+						static_assert(has_result_row_t<Rhs>::value, "argument of a union has to be a (complete) select statement");
+						static_assert(has_result_row_t<derived_statement_t<Policies>>::value, "left hand side argument of a union has to be a (complete) select statement");
+						static_assert(std::is_same<get_result_row_t<derived_statement_t<Policies>>, get_result_row_t<Rhs>>::value, "both select statements in a union have to have the same result columns (type and name)");
+
 
 						return _union_impl<void, all_t>(_check<derived_statement_t<Policies>, Rhs>{}, rhs);
 					}
diff --git a/tests/UnionTest.cpp b/tests/UnionTest.cpp
index 61436b7d..b4462ded 100644
--- a/tests/UnionTest.cpp
+++ b/tests/UnionTest.cpp
@@ -37,12 +37,12 @@ int main()
 	test::TabBar t;
 	test::TabFoo f;
 
-	db(select(t.alpha).from(t).union_distinct(select(f.epsilon).from(f)));
-	db(select(t.alpha).from(t).union_all(select(f.epsilon).from(f)));
+	db(select(t.alpha).from(t).union_distinct(select(f.epsilon.as(t.alpha)).from(f)));
+	db(select(t.alpha).from(t).union_all(select(f.epsilon.as(t.alpha)).from(f)));
 
-	auto u = select(t.alpha).from(t).union_all(select(f.epsilon).from(f)).as(sqlpp::alias::u);
+	auto u = select(t.alpha).from(t).union_all(select(f.epsilon.as(t.alpha)).from(f)).as(sqlpp::alias::u);
 
-	db(select(all_of(u)).from(u).union_all(select(t.delta).from(t)));
+	db(select(all_of(u)).from(u).union_all(select(t.delta.as(t.alpha)).from(t)));
 
 	return 0;
 }