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Active Record Associations in Rails

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Introduction

2:52 with Jay McGavren

Our blog app doesn't allow users to tell us how awesome our posts are. This situation is, of course, unacceptable. So we're going to need a comments system. We need to be able to associate comments with a particular post.

Course Prerequisites

Make sure you're familiar with all the concepts in our Rails Routes and Resources course before attempting this course.

A bit of knowledge about SQL JOIN queries will be helpful as well. You can learn more in our Querying Relational Databases course.

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    [MUSIC] 0:00
    Our blog app doesn't allow users to tell us how awesome our posts are. 0:04
    This situation is of course unacceptable. 0:08
    So we're going to need a comment system. 0:12
    But this introduces a problem. 0:14
    How do we store the comments in the database? 0:17
    Do we add additional columns to the post table to hold them? 0:19
    How many do we add? 0:22
    If we add just five columns for 0:24
    comments, any given post will be able to have a maximum of five comments. 0:25
    If we add 50 columns, most of those columns will sit empty most of the time. 0:30
    We could create a separate comments table where each record holds one comment. 0:34
    Then we could have as many or as few comments as we want. 0:38
    But then how do we show that comments belong to a particular post? 0:41
    Do we just show a big lump of comments on the front page 0:45
    that belong to the blog as a whole? 0:48
    That doesn't sound very useful. 0:50
    What we need to do is associate comments with a particular post. 0:52
    We can do this by adding a column to the comments table that refers 0:57
    back to particular records in the post table. 1:00
    All the records in comments that have a value of 1 in this new post ID 1:03
    column will refer back to the record with an ID of 1 in the post table. 1:07
    All the records in comments that have a post ID of 3 1:11
    will refer to the record with an ID of 3 in the posts table. 1:14
    All the records with post ID of 7 will refer to the post with an ID of 7 1:18
    and so on. 1:22
    This probably sounds like keeping track of all these IDs that associate records with 1:23
    each other will be a huge pain. 1:28
    Don't worry, the Active Record library that comes with Rails will 1:30
    manage everything for you, automatically. 1:33
    We just have to set up the tables and then tell Active Record 1:36
    that the post model class is associated with a comment model class. 1:39
    From then on, we can manage the creation, reading, updating, and 1:43
    deleting of comments through methods on post objects. 1:47
    And Active Record will handle all the messy database-related 1:51
    stuff that associates the comment with the post. 1:54
    Sounds cool, right? 1:58
    This is yet another way that Rails can save you a lot of work. 1:59
    In this course, we're going to show you how to configure and 2:02
    use active record associations. 2:05
    This course assumes that you've gone through all the prerequisites 2:08
    listed on the course page. 2:11
    You need to already be familiar with Rails models, views, controllers, and routes. 2:13
    A bit of knowledge about SQL join queries will be helpful as well. 2:18
    We've included links to resources where you can learn more in the teacher's notes. 2:22
    Notes for later videos in the course will have links to other relevant information. 2:26
    So if at any point you encounter something you don't understand, 2:30
    check the teacher's notes, the solution is probably there. 2:33
    And don't hesitate to use the video playback controls if you need to. 2:38
    You can adjust the playback speed to be faster or 2:42
    slower, depending on how you want to view the current content. 2:44
    Okay, ready? 2:48
    Let's set up your first Active Record Association. 2:49

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