How to select a single record in a left join

 SELECT TOP 1 M.ModelID, M.Model, C.Content
 FROM   Models M LEFT JOIN Models_Content C ON M.ModelID =  C.ModelID
 WHERE      M.ModelID = 5
 ORDER BY C.ContentID ASC

Sean's answer is the best specific solution but just to add another "generalised" solution

SELECT M.ModelID,
       M.Model,
       C.Content
FROM   Models M
       OUTER APPLY (SELECT TOP 1 *
                    FROM   Models_Content C
                    WHERE  M.ModelID = C.ModelID
                    ORDER  BY C.ContentID ASC) C
WHERE  M.ModelID = 5  

Change your JOIN to:

LEFT JOIN (SELECT ModelID, MAX(Content) as Content FROM Models_Content GROUP BY ModelID)

This is assuming you don't care which Content you get.

 SELECT
   M.ModelID, M.Model, C.Content
 FROM
   Models M
 LEFT JOIN
   Models_Content C
     ON C.ContentID = (SELECT MIN(ContentID) FROM Models_Content WHERE ModelID = M.ModelID)
 WHERE
   M.ModelID = 5

Or

;WITH sorted_content AS
(
  SELECT
    ROW_NUMBER() OVER (PARTITION BY ModelID ORDER BY ContentID) AS itemID,
    *
  FROM
    Models_Content
)
 SELECT
   M.ModelID, M.Model, C.Content
 FROM
   Models M
 LEFT JOIN
   sorted_content C
     ON  C.ModelID = M.ModelID
     AND C.itemID  = 1
 WHERE
   M.ModelID = 5