sqlite: Add a type to handle foreign keys.

This is more convenient to set a FK to null when it's unset, rather than
having to write 2 different insert statements

src/File.cpp

@@ -53,17 +53,8 @@ FilePtr File::create( DBConnection dbConnection, const std::string& mrl, unsigne
     static const std::string req = "INSERT INTO " + policy::FileTable::Name +
             "(mrl, folder_id) VALUES(?, ?)";
 
-    //FIXME: Consider having a ForeignKey type that will handle the '0' special case
-    if ( folderId != 0 )
-    {
-        if ( _Cache::insert( dbConnection, self, req, mrl, folderId ) == false )
-            return nullptr;
-    }
-    else
-    {
-        if ( _Cache::insert( dbConnection, self, req, mrl, nullptr ) == false )
-            return nullptr;
-    }
+    if ( _Cache::insert( dbConnection, self, req, mrl, sqlite::ForeignKey( folderId ) ) == false )
+        return nullptr;
     self->m_dbConnection = dbConnection;
     return self;
 }

src/Folder.cpp

@@ -53,16 +53,8 @@ FolderPtr Folder::create(DBConnection connection, const std::string& path, unsig
     auto self = std::make_shared<Folder>( path, parent );
     static const std::string req = "INSERT INTO " + policy::FolderTable::Name +
             "(path, id_parent) VALUES(?, ?)";
-    if ( parent == 0 )
-    {
-        if ( _Cache::insert( connection, self, req, path, nullptr ) == false )
-            return nullptr;
-    }
-    else
-    {
-        if ( _Cache::insert( connection, self, req, path, parent ) == false )
-            return nullptr;
-    }
+    if ( _Cache::insert( connection, self, req, path, sqlite::ForeignKey( parent ) ) == false )
+        return nullptr;
     self->m_dbConection = connection;
     return self;
 }

src/database/SqliteTools.h

@@ -14,6 +14,12 @@
 namespace sqlite
 {
 
+struct ForeignKey
+{
+    constexpr ForeignKey(unsigned int v) : value(v) {}
+    unsigned int value;
+};
+
 template <typename T, typename Enable = void>
 struct Traits;
 
@@ -27,6 +33,17 @@ struct Traits<T, typename std::enable_if<std::is_integral<T>::value>::type>
     int (*Load)(sqlite3_stmt *, int) = &sqlite3_column_int;
 };
 
+template <>
+struct Traits<ForeignKey>
+{
+    static int Bind( sqlite3_stmt *stmt, int pos, ForeignKey fk)
+    {
+        if ( fk.value != 0 )
+            return Traits<unsigned int>::Bind( stmt, pos, fk.value );
+        return sqlite3_bind_null( stmt, pos );
+    }
+};
+
 template <>
 struct Traits<std::string>
 {