如何使用 Fluent 在 Vapor 3 中从同一个 table 进行两个 JOIN 查询？

Question

这就是我想要做的（使用 postgresql）：

select H."name", A."name"

来自"Matches" M

加入"Teams" H ON M."homeTeamID" = H.id

加入"Teams" M."awayTeamID" = A.id

//This will give me an error

return Matches.query(on: request)
.join(\Teams.id, to: \Matches.homeTeamID)
.alsoDecode(Teams.self)
.join(\Teams.id, to: \Matches.awayTeamID)
.alsoDecode(Teams.self)

这里是错误：

{

错误：正确，

原因：“table 名称 "Teams" 指定了不止一次”

}

感谢任何帮助！谢谢！

Answer 1

@arema，我尝试重现您的用例，并且在 Fluent 中遇到了类似的问题。我在 Fluent 的 github 上报告了这个问题： https://github.com/vapor/fluent/issues/563

这是一个解决方法，但它远非优雅。

// Requires conforming `Match` to hashable, Equatable.
func getMatches2Handler(_ req: Request) throws -> Future<[MatchObjects]> {
    return map(
        to: [MatchObjects].self,
        Match.query(on: req).join(\Team.id, to: \Match.homeTeamID).alsoDecode(Team.self).all(),
        Match.query(on: req).join(\Team.id, to: \Match.awayTeamID).alsoDecode(Team.self).all()
    ) { homeTuples, awayTuples in
        let homeDictionary = homeTuples.toDictionary()
        let awayDictionary = awayTuples.toDictionary()
        var matchObjectsArray: [MatchObjects] = []
        matchObjectsArray.reserveCapacity(homeDictionary.count)
        for (match, homeTeam) in homeDictionary {
            let awayTeam = awayDictionary[match]!
            matchObjectsArray.append(MatchObjects(match: match, homeTeam: homeTeam, awayTeam: awayTeam))
        }
        return matchObjectsArray
    }
}

//...

extension Array {
    func toDictionary<K,V>() -> [K:V] where Iterator.Element == (K,V) {
        return self.reduce([:]) {
            var dict:[K:V] = [=10=]
            dict[.0] = .1
            return dict
        }
    }
}

Answer 2

我在这里创建了一个测试项目： https://github.com/mixio/multi-join-test

Answer 3

现在意识到这是一个老问题，但我有一个类似的问题，我通过使用原始 SQL 查询的替代方法解决了它。

下面将为主队和客队名称添加额外的列。您需要创建一个 MatchObject 来解码结果并根据您的情况建立连接。

func matchObjects(_ req: Request) throws -> Future<[MatchObject]> {
    return req.withPooledConnection(to: .psql, closure: { conn in
                return conn.raw("""
                    SELECT "Matches".*, h.name as home_team_name, a.name as away_team_name
                    FROM "Matches"
                    INNER JOIN "Teams" as h ON "Matches"."homeTeamID" = h.id
                    INNER JOIN "Teams" as a ON "Matches"."awayTeamID" = a.id
                    """).all(decoding: MatchObject.self)
            })
}

如何使用 Fluent 在 Vapor 3 中从同一个 table 进行两个 JOIN 查询？

How to do two JOIN query from the same table in Vapor 3 using Fluent?

postgresql

fluent

vapor