linq to entity does not recognize the method - asp.net-mvc

Question

I need to call a method FormatCourseTitle() from linq query but receive error message "Linq to Entity does not recognize the method FormatCourseTitle..." How to solve this problem?
public ActionResult Index()
{
var searchResults = (from a in db.Courses
join b in db.Summary on
new { subject = a.subject, catalog = a.catalog, coursetitle = FormatCourseTitle(a.coursetitle) } equals
new { subject = b.Subject, catalog = b.Catalogno, coursetitle = b.CourseTitle } into ab
from b in ab.DefaultIfEmpty()
where a.Active == true
select new JoinModel
{
Courses = a,
Summary2020 = b
} ).ToList();
return View(searResults);
}
public class JoinModel
{
[Key]
public int Id { get; set; }
public Courselist Courses { get; set; }
public Contentsummary Summary { get; set; }
}
public string FormatCourseTitle(string courseTitle)
{
//find if last three characters are " or"
string[] words = courseTitle.Trim().ToLower().Split(' ');
int j = words.Count();
string tempStr = string.Empty;
if (words[j] == "or")
{
tempStr = courseTitle.Substring(0, courseTitle.Length - 3);
}
else
{
tempStr = courseTitle;
}
return tempStr;
}

Answer 1

You should use extension method as
public static class StringHelper
{
public static string FormatCourseTitle(this string courseTitle)
{
//find if last three characters are " or"
string[] words = courseTitle.Trim().ToLower().Split(' ');
int j = words.Count();
string tempStr = string.Empty;
if (words[j] == "or")
{
tempStr = courseTitle.Substring(0, courseTitle.Length - 3);
}
else
{
tempStr = courseTitle;
}
return tempStr;
}
}
Change query to
var searchResults = (from a in db.Courses
join b in db.Summary on
new { subject = a.subject, catalog = a.catalog, coursetitle = a.coursetitle.FormatCourseTitle() } equals
new { subject = b.Subject, catalog = b.Catalogno, coursetitle = b.CourseTitle } into ab
from b in ab.DefaultIfEmpty()
where a.Active == true
select new JoinModel
{
Courses = a,
Summary2020 = b
} ).ToList();

Answer 2

I find a way to add condition in linq query to solve this problem.
var searchResults = (from a in db.Courses
join b in db.Summary on
new { subject = a.subject, catalog = a.catalog, coursetitle = a.coursetitle.Trim().EndsWith(" or")?a.coursetitle.Substring(0,a.coursetitle.Length-3):a.coursetitle } equals
new { subject = b.Subject, catalog = b.Catalogno, coursetitle = b.CourseTitle } into ab
from b in ab.DefaultIfEmpty()
where a.Active == true
select new JoinModel
{
Courses = a,
Summary2020 = b
} ).ToList();