How to check for two entities?

Question

C++ / C#

There are two entities => City and AlternativeCity.
Now the code only works for checking in the City entity

var city = await ctx.City.SingleOrDefaultAsync(x => x.CountryId == country.Id)

How to make a condition if there is no such in City, then look in AlternativeCity? And if both there and there is not, then an error?

Answer 1

Without seeing the entities themselves, it is difficult to advise something. But their fields are identical, you can try to put them in the same table and add a boolean IsAlternative field. Then query all cities and sort them by the IsAlternative field and take the first one. Thus, if there is a main city, you will get it; if there is no main city, you will get an alternative one; if null, you handle an error.
If the entities differ in structure, read about ef core inheritance, maybe this is what you need.

Answer 2

Well, if you want to get rid of lines of code, then you can collapse them into one using the second argument (defaultValue), for example (I can mess up with async/await):

ICity city = await ctx.City.SingleOrDefaultAsync(x => x.CountryId == country.Id, await ctx.AlternativeCity.SingleOrDefaultAsync(x => x.CountryId == country.Id));

If entities have a common parent interface (should in theory), then it is desirable to use it.
Also, there is a possibility that this version will be heavier due to the calculation of the default value.