search functionality on index page - search

Question

I have code with 3 search fields. One is a RadioButton and two are DropdownList coming from a database.
I have to search base on above criteria but i will get a wrong result.
public ActionResult Index(string searchby,string did, string sid)
{
ViewBag.did = new SelectList(db.destcities, "Id", "name",did);
ViewBag.sid = new SelectList(db.sourcecities, "Id", "name",sid);
var searches = db.searches.Include(s => s.destcity)
.Include(s => s.sourcecity);
If(!String.IsNullOrEmpty(searchby))
{
searches= db.searches.Where(a => a.type == searchby);
}
if(!String.IsNullOrEmpty(did))
{
int q = int.Parse(did);
searches= db.searches.Where(s => s.destcity.Id == q);
}
if (!String.IsNullOrEmpty(sid))
{
int p = int.Parse(sid);
searches= db.searches.Where(c => c.sourcecity.Id == p);
}
return View(searches);
}
and view code is
#model IEnumerable<Web.Models.search>
#{
ViewBag.Title = "Index";
}
<h2>Index</h2>
<p>#Html.ActionLink("Create New", "Create") </p>
#using (Html.BeginForm("Index","searches",FormMethod.Get))
{
<label>Trip type</label>
#Html.RadioButton("searchby", "Oneway")<text>Oneway</text>
#Html.RadioButton("searchby", "Roundtrip")<text>Roundtrip</text>
#Html.DropDownList("did", null, htmlAttributes: new { #class = "form-control" })
#Html.DropDownList("sid", null, htmlAttributes: new { #class = "form-control" })
<input type="submit" value="Create" class="btn btn-default" />
}
<table class="table">
<tr>
<th>#Html.DisplayNameFor(model => model.destcity.name)</th>
<th>#Html.DisplayNameFor(model => model.sourcecity.name)</th>
<th>#Html.DisplayNameFor(model => model.vehicle)</th>
<th>#Html.DisplayNameFor(model => model.km)</th>
<th>#Html.DisplayNameFor(model => model.price)</th>
<th>#Html.DisplayNameFor(model => model.type)</th>
<th></th>
</tr>
#foreach (var item in Model)
{
<tr>
<td>#Html.DisplayFor(modelItem => item.destcity.name)</td>
<td>#Html.DisplayFor(modelItem => item.sourcecity.name)</td>
<td>#Html.DisplayFor(modelItem => item.vehicle)</td>
<td>#Html.DisplayFor(modelItem => item.km)</td>
<td>#Html.DisplayFor(modelItem => item.price)</td>
<td>#Html.DisplayFor(modelItem => item.type)</td>
<td>#Html.ActionLink("Edit", "Edit", new { id=item.Id })
| #Html.ActionLink("Details", "Details", new { id=item.Id })
| #Html.ActionLink("Delete", "Delete", new { id=item.Id })
</td>
</tr>
}
</table>