from collections import defaultdict

def group_by_category(items):
    """Groups a list of items by their 'category' field.

    Args:
        items (list of dict): A list where each dictionary represents an item
                              and has a 'category' key.

    Returns:
        defaultdict: A dictionary where keys are categories and values are
                     lists of items belonging to that category.
    """
    grouped_data = defaultdict(list) # Initialize with list as default factory
    for item in items:
        category = item.get('category', 'uncategorized') # Handle missing category gracefully
        grouped_data[category].append(item)
    return grouped_data

# Example Usage:
products = [
    {'id': 1, 'name': 'Laptop', 'category': 'Electronics', 'price': 1200},
    {'id': 2, 'name': 'Keyboard', 'category': 'Electronics', 'price': 75},
    {'id': 3, 'name': 'Desk Chair', 'category': 'Furniture', 'price': 250},
    {'id': 4, 'name': 'Monitor', 'category': 'Electronics', 'price': 300},
    {'id': 5, 'name': 'Table', 'category': 'Furniture', 'price': 150},
    {'id': 6, 'name': 'Mouse', 'category': 'Electronics', 'price': 25}
]

organized_products = group_by_category(products)
# organized_products will be:
# defaultdict(<class 'list'>, {
#     'Electronics': [
#         {'id': 1, 'name': 'Laptop', 'category': 'Electronics', 'price': 1200},
#         {'id': 2, 'name': 'Keyboard', 'category': 'Electronics', 'price': 75},
#         {'id': 4, 'name': 'Monitor', 'category': 'Electronics', 'price': 300},
#         {'id': 6, 'name': 'Mouse', 'category': 'Electronics', 'price': 25}
#     ],
#     'Furniture': [
#         {'id': 3, 'name': 'Desk Chair', 'category': 'Furniture', 'price': 250},
#         {'id': 5, 'name': 'Table', 'category': 'Furniture', 'price': 150}
#     ]
# })
print(organized_products['Electronics'])