Support inserting many nested active models

[raffomania]

I'm inserting a graph with many nodes and edges into the database. Here's my current approach:

let node_models = Vec::new();
for node in nodes {
  node_models.push(db::nodes::ActiveModel { ... });
}

let edge_models = Vec::new();
for edge in edges {
  edge_models.push(db::edges::ActiveModel { ... });
}

db::nodes::Entity::insert_many(node_models);
db::edges::Entity::insert_many(edge_models);

Now I would like to refactor the code to use nested active models. Here's how that could work:

let node_active_models = HashMap::new();
for node in nodes {
  node_active_models.insert(node.id, db::nodes::ActiveModelEx { ... });
}

for edge in edges {
  let source_node_model = node_active_models.remove(edge.source_node_id);
  let target_node_model = node_active_models.get(edge.target_node_id);
  node_active_models.insert(source_node_model.add_target(target_node_model));
}

// This doesn't work: it converts ActiveModelEx to ActiveModel, so the edges are not inserted
db::nodes::Entity::insert_many(node_active_models);

I'd need something like an Entity::insert_many_active_model_ex function that creates a model tree with multiple root models.