cast_assoc creates or replaces the changes for an associated struct on a changeset, using a changeset function (which should call cast)īe careful with put_assoc and cast_assoc if you're dealing with a collection.put_change manually adds a change to a changeset.put_assoc creates or replaces the changes for an associated struct on a changeset.build_assoc is very useful for creating a struct associated to an existing record.There's more than one way to handle associations You can specify a different changeset function to use by passing in an optional with: &your_changeset_function/2 parameter to cast_assoc. Here's an example from the Bookmark module: defmodule Linkly.Bookmark doĭef changeset(%Bookmarkīy default, cast_assoc will use the changeset function Link.changeset to cast the association since :link was the name of the association. To do this, just include them in the cast filter in your changesets and pass in whatever data you need. The most straight-forward approach to handle records with associations is to work with their foreign keys directly. Before we finish this Ecto Beginner series, let's look at the options.
#Elixir ecto how to#
The controller is concise and does little more than present the data - as it should.One area where Ecto gives you a lot of choices is when deciding how to creating and updating records with associations to other schemas. reduce ( criteria, query, & compose_query / 2 ) end defp compose_query (, conn ) do render ( conn, ErrorView, "error.json", reason : reason ) end defp render_result ( posts, conn ) when is_list ( posts ) do render ( conn, "index.json", posts : posts ) end end Now that we know the rules around looking up a Post let’s see them applied with query composition: defp build_query ( query, criteria ) do Enum. Simple comparison is available for draft and id It's perfect in our case, since it's natural to create a new link when creating a new bookmark. While buildassoc creates a new association on a record, putassoc creates or replaces an association on a changeset. Searches for title are expected to be ILIKE "%title%" It takes a changeset, an association name and attributes. So how might we approach this problem instead?īefore we discuss the new approach let’s decide on some business rules for Post look up, see them applied in our approach, and then walk through it.įor our example we will assume the following are always true: into ( criteria, ) where ( query, , expr ) end With that in mind it’s easy to understand why the following is not only a bad idea, because it doesn’t filter the criteria, but the resulting queries are basic = comparisons: defp build_query ( query, criteria ) do expr = Enum. It’s unlikely we’ll want to search by exact title, so instead of p.title = "Repo" we want p.title ILIKE "%Repo%". Let’s consider how we might look up a blog post by title. There’s a good chance the resulting query we want won’t just be simple = comparisons. Next we’ll need to build upon base_query/0 by applying our criteria, this is where the magic of our query composition shines! When base_query/0 is called we’ll create the initial query that will serve as the base for our criteria.Īt this point our query is analogous to SELECT * FROM posts. defmodule Posts do import Ecto.Query defp base_query do from p in Post end end Let’s move ahead with creating our module and addressing the first step above: the base query. To keep things clean we’ll create a new module to contain the functionality for accessing the underlying schema data.
#Elixir ecto code#
Let’s plan to approach our composition in 3 steps:įor our example we’ll be working with everyone’s favorite example project: a blog!īefore we begin, take a peek at the schema we’ll be building our code to interface with:
In this blog post we’ll look at how we can dynamically build our Ecto queries and sanitize our input data at the same time. Ecto is fantastic tool that provides us with a great degree of flexibility.
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