Python 合并期间仅向第一个组合添加值

我有两个dfs:

df_1

date            id          value
2021-01-01      A1          100
2021-01-01      A1          200
2021-01-01      A1          300
2021-01-02      A1          100
2021-01-02      A1          200
2021-01-03      A1          500
2021-01-03      A1          800

df_2

date            id          value_to_add
2021-01-01      A1          150 
2021-01-03      A1          350 

我试图保持df_1的 struct ，并在合并过程中在第一次出现时添加value_to_add，以便在用0填充NaN和除第一个值以外的所有值后，最终结果如下所示:

date            id          value       value_to_add
2021-01-01      A1          100         150 
2021-01-01      A1          200         0               # 0 because the 150 have been already added
2021-01-01      A1          300         0
2021-01-02      A1          100         0               # 0 because value_to_add does not exist
2021-01-02      A1          200         0
2021-01-03      A1          500         350 
2021-01-03      A1          800         0               # 0 because the 350 have been already added

我的第一个 idea 是删除['date', 'id']个子集的副本，然后将df_2合并到其中，但我不确定如何回到df_1的原始 struct .

所以问题是以下-being able to merge on the first occurrence of keys during 100 operation.我在这个话题上找不到任何东西，坦率地说，我不确定如何才能做到这一点.