I have two dataframes, which both have an `Order ID` and a `date`. 

I wanted to add a flag into the first dataframe `df1`: if a record with the same `order id` and `date` is in dataframe `df2`, then add a `Y`:

```
[ df1['R'] = np.where(orders['key'].isin(df2['key']), 'Y', 0)]

```

To accomplish that, I was going to create a key, which would be the concatenation of the `order_id` and `date`, but when I try the following code:

```
df1['key']=df1['Order_ID']+'_'+df1['Date']

```

I get this error

```
ufunc 'add' did not contain a loop with signature matching types dtype('S21') dtype('S21') dtype('S21')

```

df1 looks like this: 

```
Date | Order_ID | other data points ... 
201751 4395674  ...
201762 3487535  ...

```

These are the datatypes:

```
df1.info()
RangeIndex: 157443 entries, 0 to 157442
Data columns (total 6 columns):
Order_ID                                 157429 non-null object
Date                                     157443 non-null int64
...
dtypes: float64(2), int64(2), object(2)
memory usage: 7.2+ MB

df1['Order_ID'].values
array(['782833030', '782834969', '782836416', ..., '783678018',
       '783679806', '783679874'], dtype=object)

```