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PORe-use parts of a linq query
 

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  1. PORe-use parts of a linq query
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    <p>We have a program that uses Linq-To-Sql and does a lot of similar queries on our tables. In particular, we have a timestamp column, and we pull the most recent timestamp to see if the table has changed since our last query. </p> <pre><code>System.Data.Linq.Binary ts; ts = context.Documents .Select(r =&gt; r.m_Timestamp) .OrderByDescending(r =&gt; r) .FirstOrDefault(); </code></pre> <p>We repeat this query often on different tables that's relevant for the current form/query whatever. What I would like to do is create a "function" or something that can repeat the last 3 lines of this query (and ideally would work on every table). Here's what I would like to write:</p> <pre><code>System.Data.Linq.Binary ts; ts = context.Documents .GetMostRecentTimestamp(); </code></pre> <p>But I have no idea how to create this "GetMostRecentTimestamp". Also, these queries are never this simple. They usually filter by the Customer, or by the current order, so a more valid query might be </p> <pre><code>ts = context.Documents .Where(r =&gt; r.CustomerID == ThisCustomerID) .GetMostRecentTiemstamp(); </code></pre> <p>Any help? Thanks!</p> <hr> <h3>Update [Solution]</h3> <p>I selected Bala R's answer, here's the code updated so it compiles:</p> <pre><code>public static System.Data.Linq.Binary GetMostRecentTimestamp(this IQueryable&lt;Data.Document&gt; docs) { return docs .Select(r =&gt; r.m_Timestamp) .OrderByDescending(r =&gt; r) .FirstOrDefault(); } </code></pre> <p>The only drawback to this solution is that I will have to write this function for each table. I would have loved Tejs's answer, if it actually worked, but I'm not re-designing my database for it. Plus DateTime is a not a good way to do timestamps.</p> <hr> <h3>Update #2 (Not so fast)</h3> <p>While I can do a query such as Documents.Where( ... ).GetMostRecentTimestamp(), this solution fails if I try to do an association based query such as MyCustomer.Orders.GetMostRecentTimestamp(), or MyCustomer.Orders.AsQueryable().GetMostRecentTimestamp();</p>
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    1. USDavid
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    1. CORegarding Update#2, how does it fail?
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      1. USAmy B
    2. CORegarding "DateTime is a not a good way to do timestamps.", you might be crazy.
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      1. USAmy B
    3. COSo if someone comes in and accidently updates the servers datetime to say 2012, for just a single update, then all future queries will find that DateTime using the .Max() query. Then all my checks to see if there's newer data will say "nope, I have the latest row". I guess you could just cross your fingers and hope no one bumps the DateTime of the server.
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      1. USDavid
 

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