Create acts-as-taggable-on.md

docs/docs/going-further/acts-as-taggable-on.md

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+---
+title: Acts-as-taggable-on
+sidebar_position: 13
+---
+
+If you have an `ActiveRecord` model and you're using [acts-as-taggable-on](https://github.com/mbleigh/acts-as-taggable-on),
+chances are you might want to search on tagged fields.
+
+Suppose you have this model:
+
+```rb
+class Task < ApplicationRecord
+  acts_as_taggable_on :projects
+end
+```
+
+and you have the following two instances of `Task`:
+
+```rb
+{ id: 1, name: 'Clean up my room',        projects: [ 'Home', 'Personal' ] }
+{ id: 2, name: 'Complete math exercises', projects: [ 'Homework', 'Study' ] }
+```
+
+When you're writing a `Ransack` search form, you can choose any of the following options:
+
+```erb
+<%= search_form_for @search do |f| %>
+  <%= f.text_field :projects_name_in   %> <!-- option a -->
+  <%= f.text_field :projects_name_eq   %> <!-- option b -->
+  <%= f.text_field :projects_name_cont %> <!-- option c -->
+<% end %>
+```
+
+### Option a - match keys exactly
+
+Option `a` will match keys exactly. This is the solution to choose if you want to distinguish 'Home' from 'Homework': searching for 'Home' will return just the `Task` with id 1. It also allows searching for more than one tag at once (comma separated):
+- `Home, Personal` will return task 1
+- `Home, Homework` will return task 1 and 2
+
+### Option b - match key combinations
+
+Option `b` will match all keys exactly. This is the solution if you wanna search for specific combinations of tags:
+- `Home` will return nothing, as there is no Task with just the `Home` tag
+- `Home, Personal` will return task 1
+
+### Option c - match substrings
+
+Option `c` is used to match substrings. This is useful when you don't care for the exact tag, but only for part of it:
+- `Home` will return task 1 and 2 (`/Home/` matches both `"Home"` and `"Homework"`)