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PONHibernate Group By Many-to-Many Association
 

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    <p>I need to execute a query that joins two tables, groups by a value, and returns the group key and a count.</p> <pre><code>CREATE TABLE Message( ID INT NOT NULL , Note NVARCHAR(128)); CREATE TABLE Message_Users( MessageID INT NOT NULL , UserID NVARCHAR(128) NOT NULL); </code></pre> <p>I need to execute this query:</p> <pre><code>SELECT mu.UserID, COUNT(m.ID) FROM Message m JOIN Message_Users mu ON m.ID = mu.MessageID GROUP BY mu.UserID; </code></pre> <p>How can I do this using NHibernate? The Message table is mapped as follows:</p> <pre><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt; &lt;hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="DataModel" namespace="DataModel.Message"&gt; &lt;class name="Message" table="Message"&gt; &lt;id name="ID"&gt; &lt;generator class="native" /&gt; &lt;/id&gt; &lt;bag name="AssociatedUsers" table="Message_Users" lazy="false"&gt; &lt;key column="MessageID" /&gt; &lt;element column="UserID" type="System.String" /&gt; &lt;/bag&gt; &lt;/class&gt; &lt;/hibernate-mapping&gt; </code></pre> <p>I have looked at several other questions, but none meet quite this use case (plenty of GroupBy, but not with an association; <a href="https://stackoverflow.com/questions/3900779/nhibernate-queryover-with-manytomany">plenty of join, but not with a group by</a>)</p> <p>Two directions I have started going in:</p> <p>// First attempt</p> <pre><code>// Doesn't work as message.AssociatedUsers is a collection, so mu.Key is actually a collection var query = transactionContext.Session.Query&lt;Message&gt;(); var r = from message in query group message by message.AssociatedUsers into mu select new {UserID = mu.Key, Count = mu.Count()}; </code></pre> <p>// Second attempt</p> <pre><code>var query = transactionContext.Session.QueryOver&lt;Message&gt;(); var join = query.Right.JoinQueryOver&lt;string&gt;(_message =&gt; _message.AssociatedUsers); var select = join.Select( // Select generic parameter doesn't seem to be related to the type of the join (&lt;Message, string&gt;) Projections.Group&lt;string&gt;(... </code></pre>
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