Trying to return post count in custom component

Since that query returns a count and not a list of messages, you can't iterate over the results as if it were a list of messages. If you want to iterate over a list messages change your logic to something like this (also note that you need braces after the $ when rendering message.subject):

<#assign x= rest("2.0","/search?q=" + "SELECT id, subject FROM messages WHERE board.id = 'installation'"?url) />
<#list x.data.items as message >
<p>${message.subject}</p>
</#list>

You unfortunately can't include count(*) with other fields -- it will only return the count if you do, so you will need to make a separate query to get the count of messages.

This won't work (the query will just return the count, not the messages with 'id' and 'subject' field, so the list iteration will fail):

<#assign x= rest("2.0","/search?q=" + "SELECT count(*), id, subject FROM messages WHERE board.id = 'installation'"?url) />
<#list x.data.items as message >
<p>${message.subject}</p>
</#list>

You have to do something like this:

<#assign countX = rest("2.0", "/search?q=" + "SELECT count(*) FROM messages"?url) />
<div>count: ${countX.data.count}</div>

<#assign x= rest("2.0","/search?q=" + "SELECT id, subject FROM messages"?url) />
<#list x.data.items as message >
<p>${message.subject}</p>
</#list>

-Doug