Issue understanding Salesforce Apex Trigger equality check inside in-line SOQL - triggers

Question

I ran across this Apex Trigger code that I don't quite understand...
Id is, of course, an Id
hId is a SET
I would normally expect
"WHERE Id IN :hId", instead of
"WHERE Id = :hId"
trigger linkHusbandAndWife on h2w_c (after insert) {
Set<Id> hId = new Set<Id>();
for (h2w_c h2w : Trigger.New) {
hId.add(h2w);
}
List<Husband_c> husbandlist = [
SELECT Id Wife_Id
FROM Husband_c
WHERE Id = :hId
];
...
}
Question: How can "Id = :hId" when hId is a SET?
Is this somehow equivalent to "Id IN :hId" ?

Answer 1

Yes, the syntax for queries written in Apex is quite forgiving. Especially normal SOQL (written in [brackets], not the dynamic SOQL where you craft query from Strings) . It's more of Apex feature than SOQL, you might have been looking in wrong reference guide. The magic is in "bind variables" (:) and you can't always use same trick when making raw query via API for example.
WHERE Id = :collection is fine. Collection being a single variable of type Id, a Set or a List.
WHERE Id IN :collection has same result. And you don't even need parentheses!
This means your query can ditch the intermediary step of making a Set, less code to write:
SELECT Id, Wife_Id__c FROM Husband_c WHERE Id IN :trigger.new.
A List<sObject> will be silently "cast" to List<Id> for purposes of SOQL. I used quotes because normally you can't cast it, a sObject will never be an instance of Id. If you'd need similar behaviour in normal code you'd need something like Set<Id> mySet= new Map<Id, sObject>(mylist).keyset();.
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_SOQL_variables.htm
https://developer.salesforce.com/forums/?id=906F000000090b1IAA
https://salesforce.stackexchange.com/questions/17695/how-do-i-build-a-soql-query-for-records-in-a-list-of-ids