Slick Combining query conditions effeciently - postgresql

Question

I have two table and i want to run this query everytime.
def query(host: String, id: String, key: String, map: Map[String, List[String]) =
{
val query = (for {
t1 <- TableQuery[TableA]
t2 <- TableQuery[TableB]
if t1.host === host &&
(t2.host === host) &&
(t1.id === t2.id)
} yield
t2.name)
.result
db.run(query)
}
When host is not my-host do execute the above function as it is.
But want to add an additional check only when t1.host == 'my-host', then check if the key is present in the map, if yes add the condition t1.class in map values.
I want something like:
t1 <- TableQuery[TableA]
t2 <- TableQuery[TableB]
if t1.host === host &&
t1.host === 'my-host'
(t2.host === host) &&
(t1.id === t2.id) &&
t1.value inSet map(key)
else if t1.host === host &&
(t2.host === host) &&
(t1.id === t2.id)

Answer 1

I'm not sure if you want to construct a different query for the two cases, or write a single SQL query that handles both cases.
If it's a single query, I suggest writing out the SQL you'd expect and then it should be possible to map that to a filter (for comprehension if)
But I'm guessing you want to construct a different query based on the case of host. In that case, the pattern I'd suggest is:
construct a base query (for the things you always want to do);
add on additional filters for the case you care about; and finally
map the query to the result you need (select the right columns).
(One thing to watch out for is understanding what's happening in SQL (in the database), and what's happening in Scala (in the client). For example, where you say "I want something like:" you have difficulties because inside that for-comprehension you're already in database land. That might be possible using conditional logic in SQL, but I don't think that's what you're looking for).
To give a simplified example you might construct a basic query like this:
val baseQuery = for {
t1 <- TableQuery[TableA]
t2 <- TableQuery[TableB]
if t1.host === host && t2.host === host && t1.id === t2.id
} yield (t1, t2)
Then use that to make a new query to handle the special cases:
val queryAllowingForMyHost =
baseQuery.filterIf(host == "my-host") {
case (t1, t2) => t2.host inSet Seq("x", "y", "z")
}
I've used query.filterIf there, but you could (if you prefer) write regular Scala:
val queryAllowingForMyHost =
if (host == "my-host") {
baseQuery.filter( /* extra conditions here */ )
} else {
baseQuery
}
Lastly, add any finishing touches to the query before running it:
val query = queryAllowingForMyHost.map { case (t1, t2) => t2.name }
That's the query you'd run.