Added support for foreign key constraints (#37)

tests/sqlite/test_foreign_key.cpp

@@ -0,0 +1,58 @@
+#include <gtest/gtest.h>
+
+#include <rfl.hpp>
+#include <rfl/json.hpp>
+#include <sqlgen.hpp>
+#include <sqlgen/sqlite.hpp>
+#include <vector>
+
+namespace test_foreign_key {
+
+struct Person {
+  sqlgen::PrimaryKey<uint32_t> id;
+  std::string first_name;
+  std::string last_name;
+  int age;
+};
+
+struct Relationship {
+  sqlgen::ForeignKey<uint32_t, Person, "id"> parent_id;
+  uint32_t child_id;
+};
+
+TEST(sqlite, test_foreign_key) {
+  auto people1 = std::vector<Person>(
+      {Person{
+           .id = 0, .first_name = "Homer", .last_name = "Simpson", .age = 45},
+       Person{.id = 1, .first_name = "Bart", .last_name = "Simpson", .age = 10},
+       Person{.id = 2, .first_name = "Lisa", .last_name = "Simpson", .age = 8},
+       Person{
+           .id = 3, .first_name = "Maggie", .last_name = "Simpson", .age = 0}});
+
+  const auto relationships =
+      std::vector<Relationship>({Relationship{.parent_id = 0, .child_id = 2},
+                                 Relationship{.parent_id = 0, .child_id = 3},
+                                 Relationship{.parent_id = 0, .child_id = 4},
+                                 Relationship{.parent_id = 1, .child_id = 2},
+                                 Relationship{.parent_id = 1, .child_id = 3},
+                                 Relationship{.parent_id = 1, .child_id = 4}});
+
+  using namespace sqlgen;
+  using namespace sqlgen::literals;
+
+  const auto people = sqlite::connect()
+                          .and_then(drop<Person> | if_exists)
+                          .and_then(drop<Relationship> | if_exists)
+                          .and_then(begin_transaction)
+                          .and_then(create_table<Person>)
+                          .and_then(create_table<Relationship>)
+                          .and_then(insert(std::ref(people1)))
+                          .and_then(insert(std::ref(relationships)))
+                          .and_then(drop<Person> | if_exists)
+                          .and_then(drop<Relationship> | if_exists)
+                          .and_then(commit)
+                          .value();
+}
+
+}  // namespace test_foreign_key
+