Rails ActiveRecord joins方法中单复数表名使用规则咨询

Great question! This confusion boils down to a critical detail about how ActiveRecord's joins method works: when you pass symbols to joins, you're referencing association names defined in your Rails models, not raw database table names. The singular/plural form depends entirely on the type of association you've set up.

Let's break this down with your code example:

Core Rule: Association Type Determines Singular/Plural #

• Plural symbols: Used for has_many or has_and_belongs_to_many associations. These represent a collection of related records, so the name is pluralized.
• Singular symbols: Used for belongs_to or has_one associations. These represent a single related record, so the name is singular.

Analyzing Your Code #

Let's walk through each part of your joins calls:

1. product_attr_vals (plural)
   Your Product model almost certainly has a has_many :product_attr_vals association (since one product can have multiple attribute values). ActiveRecord looks for this plural-named association to build the join—using the singular product_attr_val would fail because no such singular association exists in your Product model.

2. attr_val (singular)
   Your ProductAttrVal model likely has a belongs_to :attr_val (or has_one :attr_val) association. Since each product attribute value links to exactly one attribute value record, the association name is singular, so you use :attr_val here.

3. :product_model (singular)
   Similarly, your Product model has a belongs_to :product_model association (one product belongs to one product model). This is a singular association, so you pass the singular symbol :product_model.

Bonus: When Would You Use Table Names? #

If you ever need to write a custom join using raw table names (instead of model associations), you'd pass a string to joins instead of symbols, like:

Product.joins("INNER JOIN product_attr_vals ON products.id = product_attr_vals.product_id")

But the symbol-based syntax is preferred because it leverages your model's associations, keeping your code DRY and aligned with Rails conventions.

内容的提问来源于stack exchange，提问作者Augusto Carmo