Question: I hope the problem is clear. Is there an alternative way not to get undefined reference error without implementing template functions in header files? Thanks in advance.

Answer: 
Your solution might be too confusing later and you may have more headaches later.

I advice this: (1) Use a NoSQL such as MongoDB. (2) Migrate all your data from previous DB. (3) Then put your DB under versioning with. You can, since it's document-based and not SQL.

Mongo has an excellent Gem too, called MongoID <https://github.com/mongoid/mongoid> 

This way you can use a CMS which has much bigger community (like Refinery). Further more: your DB backups solved easily becuase you are able to roll back anytime with Git or simply clone your DB sometimes, also you can automatize backups, etc.

HTH
Maybe you could combine a git wrapper for editing, etc:

<https://github.com/schacon/ruby-git>

with your own rendering code based on a simple file based storage system.

I'm not so familiar with Ruby though, so can't help much on the ruby rendering side of things.

This comment you make "most of my site will contain advanced dynamic functionality" looks like you are going to need to roll your own solution based on your unique requirements.