There are gems for tagging but I find for simple cases they're unnecessary.

Setup

Create two models:

rails g model tag name:text

rails g model tag_instance tag_id:integer item_id:integer item_type:string

Add relationships to your TagInstance model

belongs_to :tag
belongs_to :item, polymorphic: true

validates_uniqueness_of :tag_id, scope: [:item_id, :item_type]

Adding tag-ability to a model

Now, in any other model, to enable tagging you just have to add

has_many :tag_instances, as: :item
has_many :tags, through: :tag_instances

Usage

Add a tag to any object with this two-liner

post = Post.last
tag = Tag.find_or_create_by(name:"My Tag")
post.tags << tag unless post.tags.exists?(tag.id)

Find all objects with a particular tag with this

Post.joins(:tags).where("tags.name = ?","My Tag")