如何逐步構建LINQ => SQL /實體查詢（帶連接）？

Question

我有以下兩個LINQ查詢：

public int getJobsCount() 
{ 
    var numJobs = 
     (from j in dbConnection.jobs 
     join i in dbConnection.industries on j.industryId equals i.id 
     join c in dbConnection.cities on j.cityId equals c.id 
     join s in dbConnection.states on j.stateId equals s.id 
     join pt in dbConnection.positionTypes on j.positionTypeId equals pt.id 
     select j).Count(); 
    return numJobs; 
} 

public List<Job> getJobs() 
{ 
    var jobs = 
     (
      from j in dbConnection.jobs 
      join i in dbConnection.industries on j.industryId equals i.id 
      join c in dbConnection.cities on j.cityId equals c.id 
      join s in dbConnection.states on j.stateId equals s.id 
      join pt in dbConnection.positionTypes on j.positionTypeId equals pt.id 
      orderby j.issueDatetime descending 
      select new Job { x = j.field, y = c.field, etc } 
     ).Skip(startJob - 1).Take(numJobs); 
    return jobs; 
} 

有很多的在那裏重複的代碼 - 「從」和「加盟」線是相同的，我會在一些「其中」被添加線也是一樣的。

我想補充說，返回一個IQueryable對於第一部分的方法：

public IQueryable getJobsQuery() 
{ 
    var q = 
     from j in dbConnection.jobs 
     join i in dbConnection.industries on j.industryId equals i.id 
     join c in dbConnection.cities on j.cityId equals c.id 
     join s in dbConnection.states on j.stateId equals s.id 
     join pt in dbConnection.positionTypes on j.positionTypeId equals pt.id; 
    return q; 
} 

...但我得到「查詢體必須與SELECT子句或group子句結束」。

如果我堅持到最後關閉該功能添加一個選擇條款，我不能調用的結果數（）：

// getJobsQuery: 
var q = from j in dbConnection.jobs 
     join i in dbConnection.industries on j.industryId equals i.id 
     join c in dbConnection.cities on j.cityId equals c.id 
     join s in dbConnection.states on j.stateId equals s.id 
     join pt in dbConnection.positionTypes on j.positionTypeId equals pt.id 
     select new { a = j.y, b = c.z } 

// another method: 
var q = getJobsQuery(); 
var numJobs = q.Count(); // "IQueryable doesn't contain a definition for count" 

是否有建立這樣的查詢採取逐步的方式避免重複一大堆代碼的步驟？

Answer 1

有寫LINQ查詢兩種方式，雖然它並不重要女巫一個使用它的好，原因知道他們兩個，他們可能會學到一些關於LINQ如何工作的信息。

例如，您有一組作業。如果你選擇用5的industryId（數據類型的胡亂猜測）所有的工作你可能會寫這樣的事：

from j in dbConnection.jobs 
where j.inustryId == 5 
select j; 

非常相同的查詢也可以寫成這樣

dbConnections.jobs.Where(j => j.industryId == 5); 

現在，我不是在這裏宣揚一種方式比另一種更好，但在這裏你可以清楚地看到LINQ如何使用擴展方法語法自動選擇迭代對象（除非你做了選擇），而在查詢語法中，你必須明確地做到這一點。另外，如果你要添加的旅館這裏另一個where子句會是這個樣子：

from j in dbConnection.jobs 
where j.inustryId == 5 // not using && here just to prove a point 
where j.cityId == 3 // I THINK this is valid syntax, I don't really use the query-syntax in linq 
select j; 

雖然在擴展的方法你可以追加更多的方法，調用像這樣：

dbConnections.jobs.Where(j => j.industryId == 5) 
    .Where(j => j.cityId == 3); 

現在，這是很好的知道原因，這意味着你可以將你的linq查詢放入函數中繼續查詢。和所有你需要做的，讓你的情況的工作僅僅是明確地選擇起始變量j，或者你需要像這樣所有的變量：

var q = 
    from j in dbConnection.jobs 
    join i in dbConnection.industries on j.industryId equals i.id 
    join c in dbConnection.cities on j.cityId equals c.id 
    join s in dbConnection.states on j.stateId equals s.id 
    join pt in dbConnection.positionTypes on j.positionTypeId equals pt.id; 
      select new {j = j, i = i, c = c, s = s, pt = pt }; 
return q; 

那麼你應該能夠例如該做的事：

getJobsQuery().Where(a => a.i.id == 5); // I used a as a name for "all", like the collection of variables 

或使用查詢語法

from a in getJobsQuery() 
where a.i.id == 5 
select a; 

Answer 2

通過返回一組數據（例如公共數據）並查詢該數據的子集可以更好地解決這個問題嗎？

E.g. [僞]

var allJobs = 
    (from j in dbConnection.jobs 
    join i in dbConnection.industries on j.industryId equals i.id 
    join c in dbConnection.cities on j.cityId equals c.id 
    join s in dbConnection.states on j.stateId equals s.id 
    join pt in dbConnection.positionTypes on j.positionTypeId equals pt.id 
    select j); 

var myJobs = allJobs.OrderBy(j => j.issuedate).skip(expr).Take(allJobs.Count); 

或類似...