[odb-users] "Access violation reading location" when querying a view

[Seiler Matthias SIGCH]

Hi Boris,

I have the following view ...

```
#pragma db view query(                  \
    "SELECT TpEntity.Id "               \
    "FROM TpEntity "                    \
    "INNER JOIN Switch "                \
    "ON Switch.TpEntity = TpEntity.Id " \
    "WHERE (?)")
struct TpEntityIdBySwitch {
  unsigned long tpEntityId;
};
```

... which is queried like this:

```
transaction t(db->begin());
auto q = "Switch.Id='" + switchId + "'";
auto viewRes = *db->query_one<TpEntityIdBySwitch>(q);
t.commit();
```

Thus, given a `Switch.Id` (whose table has a foreign key to `TpEntity` on `Switch.TpEntity`), I want to get the value of `TpEntity.Id`. It works fine if the passed ID exists. However, if it does not, I get an "Access violation reading location ...", which I'm not able to catch. How can I go around this issue such that it does not crash, maybe getting "some" result (e.g., NULL or optional) regardless of if the table contains the ID?

Kind regards,
Matthias