![]() ![]() ![]() The current state with the transition requested and perform the required transformations. By sending appropriate events, we are able to match This implements a process that starts out in the :locked state. start_link ( _MODULE_, :ok, ) end :gen_statem def init ( _ ), do: end end We could model this as a finite-state machine, such as the following:ĭefmodule Door do :gen_statem def start_link do :gen_statem. e.g: Tasks start out in the backlog, can be assigned to people, moved to "in progress", and later to "done"įor this post, we'll stick with a small example that illustrates the flow of a state machine: A door.Ī door can be locked or unlocked. A task in a project management pipeline.A shopping cart which starts out empty, allows products to be added indefinitely and can proceed to payment/shipping if enough products are added.A user onboarding flow where the user first signs up, then adds some extra required info, confirms his email, then enables 2FA, and only then is allowed into the system.Requirements, a state machine can be a good choice as an abstraction. When you model a long-running flow that requires multiple steps, and where each step has different In this post, you'll learn how to implement this pattern with Elixir's Ecto and when to use it. It's a great abstraction in many different scenarios, where you want to model a certain process that goes through a predefined set of states, with different behaviors, depending on what state it is in. The controller is concise and does little more than present the data - as it should.Among the many useful patterns in computer science, there is the concept of a Finite-state machine (FSM). 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 Searches for title are expected to be ILIKE "%title%" 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. 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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