将巧妙的查询组合成单个查询

Question

如何使用 Slick 3.1 将多个查询合并为同一类型的单个查询？这 不是联接或联合 ，而是组合查询 "segments" 以创建单个查询请求。这些 "segments" 可以是任何单独有效的查询。

val query = TableQuery[SomeThingValid]

// build up pieces of the query in various parts of the application logic
val q1 = query.filter(_.value > 10)
val q2 = query.filter(_.value < 40)
val q3 = query.sortBy(_.date.desc)
val q4 = query.take(5)

// how to combine these into a single query ?
val finalQ = ??? q1 q2 q3 q4 ???

// in order to run in a single request
val result = DB.connection.run(finalQ.result)

编辑：预期的 sql 应该是这样的：

SELECT * FROM "SomeThingValid" WHERE "SomeThingValid"."value" > 10 AND "SomeThingValid"."valid" < 40 ORDER BY "MemberFeedItem"."date" DESC LIMIT 5

Answer 1

我已经在 slick2.0

中使用了这个 "pattern"

val query = TableQuery[SomeThingValid]

val flag1, flag3 = false
val flag2, flag4 = true

val bottomFilteredQuery = if(flag1) query.filter(_.value > 10) else query
val topFilteredQuery = if(flag2) bottomFilteredQuery.filter(_.value < 40) else bottomFilteredQuery
val sortedQuery = if(flag3) topFilteredQuery.soryBy(_.date.desc) else topFilteredQuery
val finalQ = if(flag4) sortedQuery.take(5) else sortedQuery

Answer 2

我想，应该可以。但是我还没有测试。

val users = TableQuery[Users]
val filter1: Query[Users, User, Seq] = users.filter(condition1)
val filter2: Query[Users, User, Seq] = users.filter(condition2)
(filter1 ++ filter2).result.headOption

Answer 3

val q1 = query.filter(_.value > 10)
val q2 = q1.filter(_.value < 40)
val q3 = q2.sortBy(_.date.desc)
val q4 = q3.take(5)

我认为你应该像上面那样做（并传递 Querys）但是如果你坚持传递查询 "segments"，这样的事情可能会起作用：

type QuerySegment = Query[SomeThingValid, SomeThingValid, Seq] => Query[SomeThingValid, SomeThingValid, Seq]

val q1: QuerySegment = _.filter(_.value > 10)
val q2: QuerySegment = _.filter(_.value < 40)
val q3: QuerySegment = _.sortBy(_.date.desc)
val q4: QuerySegment = _.take(5)

val finalQ = Function.chain(Seq(q1, q2, q3, q4))(query)

Answer 4

这本华而不实的必备书籍值得一提，看来您可能需要避免在一个查询中组合多个查询。

Combining actions to sequence queries is a powerful feature of Slick. However, you may be able to reduce multiple queries into a single database query. If you can do that, you’re probably better off doing it.

将巧妙的查询组合成单个查询

combining slick queries into single query

scala

slick-3.0