Jonathan goes to Castle Dracula

Capturing a SELECT expression

With these countries added, we are now ready to make Dracula. First we will change places_visited in Person from City to Place so that it can include many things: London, Bistritz, Hungary, etc. We only know that Dracula has been in Romania, so we can do a quick FILTER when we select it. When doing this, we put the SELECT inside () brackets. The brackets (parentheses) are necessary to capture the result of the query using SELECT, that we then use to do something. In other words, the brackets delimit (set the boundaries for) a set. EdgeDB will do the operation inside the brackets, and then that completed result is given to places_visited.

INSERT Vampire {
  name := 'Count Dracula',
  places_visited := (SELECT Place FILTER .name = 'Romania'),
  # .places_visited is the result of this SELECT query.
};

The result is {Object {id: 0a1b83dc-f2aa-11ea-9f40-038d228e2bba}}.

The uuid there is the reply from the server showing which object was just created and that we were successful.

Let’s check if places_visited worked. We only have one Vampire object now, so let’s SELECT it:

SELECT Vampire {
  places_visited: {
    name
  }
};

This gives us: {Object {places_visited: {Object {name: 'Romania'}}}}

Perfect.

Adding constraints

Now let’s think about age again. It was easy for the Vampire type, because they can live forever. But now we want to give age to PC and NPC too, who are humans who don’t live forever (we don’t want them living up to 32767 years). For this we can add a “constraint” (a limit). Instead of age, we’ll give them a new type called HumanAge. Then we can write constraint on it and use one of the functions that it can take. We will use max_value().

Here’s the signature for max_value():

std::max_value(max: anytype)

The anytype part is interesting, because that means it can work on types like strings too. With a constraint max_value('B') for example you couldn’t use ‘C’, ‘D’, etc.

Now let’s go back to our constraint for HumanAge, which is 120. The HumanAge type looks like this:

scalar type HumanAge extending int16 {
  constraint max_value(120);
}

Remember, it’s a scalar type because it can only have one value. Then we’ll add it to the NPC type.

type NPC extending Person {
  property age -> HumanAge;
}

It’s our own type with its own name, but underneath it’s an int16 that can’t be greater than 120. So if we write this, it won’t work:

INSERT NPC {
    name := 'The innkeeper',
    age := 130
};

Here is the error: ERROR: ConstraintViolationError: Maximum allowed value for HumanAge is 120. Perfect.

Now if we change age to 30, we get a message showing that it worked: {Object {id: 72884afc-f2b1-11ea-9f40-97b378dbf5f8}}. Now no NPCs can be over 120 years old.

Deleting objects

Deleting in EdgeDB is very easy: just use the DELETE keyword. It’s similar to SELECT in that you write DELETE and then the type, which will by default delete them all. And in the same way as SELECT, if you FILTER then it will only delete the ones that match the filter.

This similarity to SELECT might make you nervous, because if you type something like SELECT City then it will select all of them. DELETE is the same: DELETE City deletes every object for the City type. That’s why a confirmation message pops up if you delete without FILTER to make sure that you really want to delete everything. But it won’t confirm if you use FILTER, because it deletes fewer things and assumes that you know exactly what you want to delete.

So let’s give it a try. Remember our two Country objects for Hungary and Romania? Let’s delete them:

DELETE Country;

Just like an insert, it gives us the id numbers of the objects that are now deleted:

{Object {id: bc9c4766-2898-11eb-b5e8-0bfd25af166c}, Object {id: bd0a6b1a-2898-11eb-b5e8-ef85694d442f}}

Okay, insert them again. Now let’s delete with a filter:

DELETE Country FILTER .name ILIKE '%States%';

Nothing matches, so the output is {} - we deleted nothing. Let’s try again:

DELETE Country FILTER .name ILIKE '%ania%';

We got a {Object {id: eaa9b03a-2898-11eb-b5e8-27121e63218a}}, which is certainly Romania. Only Hungary is left. What if we want to see what we deleted? No problem - just put the DELETE inside brackets and SELECT it. Let’s delete all the Country objects again but this time we’ll select it:

SELECT (DELETE Country) {
  name
};

The output is {Object {name: 'Hungary'}}, showing us that we deleted Hungary. And now if we do SELECT Country we get a {}, which confirms that we did delete them all.

(Fun fact: DELETE statements in EdgeDB are actually syntactic sugar for DELETE (SELECT ...). You’ll be learning something called LIMIT in the next chapter with SELECT and as you do so, keep in mind that you can apply the same to DELETE too.)

Finally, let’s insert Hungary and Romania again to finish the chapter. We’ll leave them alone now.

Here is all our code so far up to Chapter 3.

1. This query is trying to display every NPC along with the name plus every City type for each NPC, but it’s giving an error. What is it missing?

   Copy

   SELECT NPC {
     name,
     cities := SELECT City.name
   };
   

   Show answer

2. If the City type needed a required property called population, what would it look like? What type would ‘population’ be?

   Show answer

3. This query wants to display name twice for some reason but is giving an error. Can you think of a way to do it?

   Copy

   SELECT Person {
     name,
     name
   };
   

   (Hint: the problem is that the name name is being used twice)

   Show answer

4. People keep trying to make characters with negative ages. Can you think of a constraint that can stop this?

   Hint: the current constraint is max_value(120);

   Show answer

5. Can you insert a HumanAge type?

   Show answer

Up next: Jonathan: “This Count Dracula knows so much about history! I’m glad I came.”