Storing a List in a Blob with sqflite - flutter

Question

I am looking for an example of storing a generic Dart object (typically a List<MyObject>) in a Blob column with sqflite (and retrieving it back please) assuming it will store it as binary data.
MyObject is an object used by my application, its content is not important here as I am looking for a generic/universal method.
Could anybody be kind enough to provide a snippet?
Documentation on this subject is rare with Dart. Closest I could find would include marshaling the object as Uint8List something like this or something like that but I could not really understand/apply them.
I have found this similar issue, where the developer ends up converting his lists as a string... We really cannot do better (ie more efficient) ?
Many thanks for your help.
Patrick

Answer 1

You need to convert MyObject into a serialized stream of bytes using BytesBuilder() and then insert into a sqflite BLOB as an Uint8List.
For a List there are several strategies:
If the size of MyObject is the same for every element in the list,
just store them one after another.
If the size is variable, store
first the number of bytes for each one and then the object itself,
repeating for all elements on the list.
Or store each MyObject
followed by a separator. The separator must be chosen to be unique:
no separator shall be part of any MyObject instances.
Normally, the second option is more robust and flexible. Example:
MyList = getTheListOfMyObject();
final bytes = BytesBuilder();
for(int i = 0; i < myList.length; i++) {
bytes.addByte(sizeOfMyObject[i]);
// Here starts the serialization of MyObject
// only you know how it is structured, but
// every member could be expressed as series of bytes
// and remember to think on how to deserialize them
// in order to recover MyObject from the sqflite BLOB
bytes.addByte(firstByteOfMyObject[i]);
bytes.addByte(secondByteOfMyObject[i]);
bytes.addByte(thirdByteOfMyObject[i]);
// and so on.
}
// Then retrieve the full stream as a Uint8List:
final blob = bytes.toBytes();
Map<String, dynamic> map = {
'id': 1,
'my_list': blob,
};
res = await _db.insert("my_table", map);
The table for the example: "CREATE TABLE my_table(id INTEGER, my_list BLOB)"
It is a better coding practice to implement the serialization/deserialization of MyObject inside of MyObject class. The database routines shall retrieve the Uint8List from a getter before storing it and shall pass the bytes to a setter for the class to reconstruct the object structure. That way you keep the implementation details for the object properly encapsulated.