理解Rails中与ActiveRecord的关系

Question

I understand SQL I understand Database Methodology I understand Rails I don't understand ActiveRecord Relations

I have three models (simplified) that i created with rails g model and then performed a rake db:migrate with a mysql db.

Calendar
id: number
name: string

Event
id: number
title: string

Event_Tag
id: number
event_id
key: string
value: string

How would i perform a query for all events and retrieve all the event_tags for each so that i could build a JSON model with minimal queries?

Event
id: number
title: string
tags: [
    <key>:<value>,
    <key>:<value>,
    <key>:<value>,
]

I have tried this:

events = Event.includes(:event_tag).where("events.calendar_id" => params[:calendar_id])

Which i have seen avoids the N+1 query issue. And it performs 2 queries.

Event Load (47.1ms)  SELECT `events`.* FROM `events`  WHERE `events`.`calendar_id` = 1

EventTag Load (46.3ms)  SELECT `event_tags`.* FROM `event_tags`  WHERE `event_tags`.`event_id` IN (1) 

But when i inspect an event

eventHash = events[0]

i receive

#<ActiveRecord::Relation [#<Event id: 1, title: "Test", calendar_id: 1, created_at: nil, updated_at: nil>]>

Answer 1

ActiveRecord

Although I don't know the behind-the-scenes workings of ActiveRecord, I do know it's what's called an ORM (Object Related Mapping) system (framework for SQL)

This means it gives you a series of methods to call, which it will then translate into the relevant SQL queries ( joins etc)

---

Associations

ActiveRecord associations are basically glorified join statements - each time you call an object, and then reference its associated data, you will basically be calling two queries to ping two separate data tables.

The way AR works is to use the foreign_key

I think your problem is best solved using an association defined in your Event model:

#app/models/event.rb
Class Event < ActiveRecord::Base
    has_many :tags, class_name: "EventTag"
end
#app/models/event_tag.rb
Class EventTag < ActiveRecord::Base
    belongs_to :event
end

This will allow you to call the following:

@event = Event.find params[:id]
@event.tags.each do |tag|
   tag.name
   tag.value
end

---

If this helps, if you comment, I'll see about helping combine the tags into a JSON hash for you, although I'm not sure what you're trying to do with that exactly

Answer 2

The includes method in Rails eager loads the relationships into memory for quicker access so the application doesn't do all the N+1 queries.

So even though the relation doesn't show up when you inspect the code, if you access it thru the relation (eg event.event_tag.key ) the application doesn't perform additional queries since the data is already present in memory.

Here's a really good article explaining eager loading in Rails (discusses includes and other useful methods eager_load and preload and how they're all related): http://blog.arkency.com/2013/12/rails4-preloading/

As far as building the JSON object, it really depends on how you're going about that. I'd imagine the most common libraries (eg jbuilder or active_model_serializers) would handle the relations query fine.