_id  parent_id  type
    A    B          Subcategory_level
    B    null       Main_Level
    D    A          Product_Level
    M    N          Product_Level
    X    Y          Subcategory_Level
    Z    X          Subcategory_Level
    L    Z          Product_Level

_id  parent_id  type
    D    A          product_level
    M    N          product_level
    L    X          product_level

df1=df
    df1.rename(columns= {'_id':'parent_id','parent_id':'_id'},

final_df=pd.merge(df,df1,on='parent_id', how='inner')