Ecto查询中Object与%Object{}的区别是什么？

Great question! Let's break down the key differences between using Object (your Ecto schema module) and %Object{} (an instance of the schema struct) when building Ecto queries:

Core Definitions #

• Object is the schema module you defined with use Ecto.Schema. It stores all metadata about your database table—like the table name, field types, associations, and constraints. Think of it as the blueprint for your database records.
• %Object{} is an instance of the schema struct. When created this way, it initializes with default values (usually nil unless you set explicit defaults in your schema) for all fields defined in the module.

Behavior in Queries #

For the exact code you shared, both approaches will generate the same SQL query. Ecto can extract the necessary schema metadata from either the module or a struct instance, so the end result is identical here. But there are important edge cases where they diverge:

• Automatic filtering with struct values: If you create a struct with pre-set fields (e.g., %Object{parent_id: nil}) and pipe it into a query, Ecto will automatically add a WHERE parent_id IS NULL clause to your query. That's a handy shortcut! Using Object doesn't carry any pre-filled values, so you have to explicitly write the where clause like you did.
• Tiny overhead: Creating a struct (%Object{}) has minimal performance overhead compared to referencing the module directly. In almost all applications, this is completely negligible—you'd only notice a difference if you were spawning millions of structs per second.
• Readability conventions: Many Elixir teams prefer using Object for top-level queries because it makes it explicit that you're starting from the base schema, not a pre-existing record. %Object{} is more commonly used when you're building a query based on an existing struct instance (like fetching related records for a specific object).

Best Practices #

• Stick with Object for standard, top-level queries where you don't need to carry pre-filled field values. It's the most common and clear approach.
• Use %Object{} when you want to leverage Ecto's automatic filtering based on struct fields, or when you're building a query that starts from an existing struct (e.g., my_object |> assoc(:children) |> Repo.all()).

内容的提问来源于stack exchange，提问作者Borybar