Ecto is split into 4 main components:

• Ecto.Repo - repositories are wrappers around the data store. Via the repository, we can create, update, destroy and query existing entries. A repository needs an adapter and credentials to communicate to the database

• Ecto.Schema - schemas allow developers to define data structures that map to the underlying storage

• Ecto.Changeset - changesets provide a way for developers to filter and cast external parameters, as well as a mechanism to track and validate changes before they are sent to the repository

• Ecto.Query - written in Elixir syntax, queries are used to retrieve information from a given repository. Queries in Ecto are secure, avoiding common problems like SQL Injection, while still being composable, allowing developers to build queries piece by piece instead of all at once

In the following sections, we will provide an overview of those components and how they interact with each other. Feel free to access their respective module documentation for more specific examples, options and configuration.

If you want to quickly check a sample application using Ecto, please check https://github.com/elixir-lang/ecto/tree/master/examples/simple.

Repositories

Ecto.Repo is a wrapper around the database. We can define a repository as follows:

defmodule Repo do
  use Ecto.Repo, otp_app: :my_app
end

Where the configuration for the Repo must be in your application environment, usually defined in your config/config.exs:

config :my_app, Repo,
  adapter: Ecto.Adapters.Postgres,
  database: "ecto_simple",
  username: "postgres",
  password: "postgres",
  hostname: "localhost",
  # OR use a URL to connect instead
  url: "postgres://postgres:postgres@localhost/ecto_simple"

Each repository in Ecto defines a start_link/0 function that needs to be invoked before using the repository. In general, this function is not called directly, but used as part of your application supervision tree.

If your application was generated with a supervisor (by passing --sup to mix new) you will have a lib/my_app.ex file containing the application start callback that defines and starts your supervisor. You just need to edit the start/2 function to start the repo as a supervisor on your application’s supervisor:

def start(_type, _args) do
  import Supervisor.Spec

  children = [
    supervisor(Repo, [])
  ]

  opts = [strategy: :one_for_one, name: MyApp.Supervisor]
  Supervisor.start_link(children, opts)
end

Schema

Schema provide a set of functionalities around structuring your data, defining relationships and applying changes to repositories.

Let’s see an example:

defmodule Weather do
  use Ecto.Schema

  # weather is the DB table
  schema "weather" do
    field :city,    :string
    field :temp_lo, :integer
    field :temp_hi, :integer
    field :prcp,    :float, default: 0.0
  end
end

By defining a schema, Ecto automatically defines a struct with the schema fields:

iex> weather = %Weather{temp_lo: 30}
iex> weather.temp_lo
30

The schema also allows us to interact with a repository:

iex> weather = %Weather{temp_lo: 0, temp_hi: 23}
iex> Repo.insert!(weather)
%Weather{...}

After persisting weather to the database, it will return a new copy of %Weather{} with the primary key (the id) set. We can use this value to read a struct back from the repository:

# Get the struct back
iex> weather = Repo.get Weather, 1
%Weather{id: 1, ...}

# Delete it
iex> Repo.delete!(weather)
%Weather{...}

NOTE: by using Ecto.Schema, an :id field with type :id (:id means :integer) is generated by default, which is the primary key of the Model. If you want to use a different primary key, you can declare custom @primary_key before the schema/2 call. Consult the Ecto.Schema documentation for more information.

Notice how the storage (repository) and the data are decoupled. This provides two main benefits:

• By having structs as data, we guarantee they are light-weight, serializable structures. In many languages, the data is often represented by large, complex objects, with entwined state transactions, which makes serialization, maintenance and understanding hard;

• By making the storage explicit with repositories, we don’t pollute the repository with unnecessary overhead, providing straight-forward and performant access to storage;

Changesets

Although in the example above we have directly inserted and deleted the struct in the repository, update operations must be done through changesets so Ecto can efficiently track changes.

Further than that, changesets allow developers to filter, cast, and validate changes before we apply them to the data. Imagine the given schema:

defmodule User do
  use Ecto.Schema

  import Ecto.Changeset

  schema "users" do
    field :name
    field :email
    field :age, :integer
  end

  def changeset(user, params \\ %{}) do
    user
    |> cast(params, [:name, :email, :age])
    |> validate_required([:name, :email])
    |> validate_format(:email, ~r/@/)
    |> validate_inclusion(:age, 18..100)
  end
end

The changeset/2 function first invokes Ecto.Changeset.cast/3 with the struct, the parameters and a list of required and optional fields; this returns a changeset. The parameter is a map with binary keys and a value that will be cast based on the type defined on the schema.

Any parameter that was not explicitly listed in the required or optional fields list will be ignored. Furthermore, if a field is given as required but it is not in the parameter map nor in the struct, it will be marked with an error and the changeset is deemed invalid.

After casting, the changeset is given to many Ecto.Changeset.validate_*/2 functions that validate only the changed fields. In other words: if a field was not given as a parameter, it won’t be validated at all. For example, if the params map contain only the “name” and “email” keys, the “age” validation won’t run.

As an example, let’s see how we could use the changeset above in a web application that needs to update users:

def update(id, params) do
  changeset = User.changeset Repo.get!(User, id), params["user"]

  case Repo.update(changeset) do
    {:ok, user} ->
      send_resp conn, 200, "Ok"
    {:error, changeset} ->
      send_resp conn, 400, "Bad request"
  end
end

The changeset/2 function receives the user and its parameters and returns a changeset. If the changeset is valid, we persist the changes to the database, otherwise, we handle the error by emitting a bad request code.

Another example to create users:

def create(id, params) do
  changeset = User.changeset %User{}, params["user"]

  case Repo.insert(changeset) do
    {:ok, user} ->
      send_resp conn, 200, "Ok"
    {:error, changeset} ->
      send_resp conn, 400, "Bad request"
  end
end

The benefit of having explicit changesets is that we can easily provide different changesets for different use cases. For example, one could easily provide specific changesets for create and update:

def create_changeset(user, params) do
  # Changeset on create
end

def update_changeset(user, params) do
  # Changeset on update
end

Changesets are also capable of transforming database constraints, like unique indexes and foreign key checks, into errors. Allowing developers to keep their database consistent while still providing proper feedback to end users. Check Ecto.Changeset.unique_constraint/3 for some examples as well as the other _constraint functions.

Query

Last but not least, Ecto allows you to write queries in Elixir and send them to the repository, which translates them to the underlying database. Let’s see an example:

import Ecto.Query, only: [from: 2]

query = from w in Weather,
          where: w.prcp > 0 or is_nil(w.prcp),
          select: w

# Returns %Weather{} structs matching the query
Repo.all(query)

Queries are defined and extended with the from macro. The supported keywords are:

• :distinct
• :where
• :order_by
• :offset
• :limit
• :lock
• :group_by
• :having
• :join
• :select
• :preload

Examples and detailed documentation for each of those are available in the Ecto.Query module. Functions supported in queries are listed in Ecto.Query.API.

When writing a query, you are inside Ecto’s query syntax. In order to access params values or invoke Elixir functions, you need to use the ^ operator, which is overloaded by Ecto:

def min_prcp(min) do
  from w in Weather, where: w.prcp > ^min or is_nil(w.prcp)
end

Besides Repo.all/1 which returns all entries, repositories also provide Repo.first/1 which returns one entry or nil, Repo.first!/1 which returns one entry or raises, Repo.get/2 which fetches entries for a particular ID and more.

Finally, if you need a escape hatch, Ecto provides fragments (see Ecto.Query.API.fragment/1) to inject SQL (and non-SQL) fragments into queries. Also, most adapters provide direct APIs for queries, like Ecto.Adapters.SQL.query/4, allowing developers to completely bypass Ecto queries.

Other topics

Associations

Ecto supports defining associations on schemas:

defmodule Post do
  use Ecto.Schema

  schema "posts" do
    has_many :comments, Comment
  end
end

defmodule Comment do
  use Ecto.Schema

  schema "comments" do
    field :title, :string
    belongs_to :post, Post
  end
end

When an association is defined, Ecto also defines a field in the schema with the association name. By default, associations are not loaded into this field:

iex> post = Repo.get(Post, 42)
iex> post.comments
#Ecto.Association.NotLoaded<...>

However, developers can use the preload functionality in queries to automatically pre-populate the field:

Repo.all from p in Post, preload: [:comments]

Preloading can also be done with a pre-defined join value:

Repo.all from p in Post,
          join: c in assoc(p, :comments),
          where: c.votes > p.votes,
          preload: [comments: c]

Finally, for the simple cases, preloading can also be done after a collection was fetched:

posts = Repo.all(Post) |> Repo.preload(:comments)

The Ecto module also provides conveniences for working with associations. For example, Ecto.assoc/2 returns a query with all associated data to a given struct:

import Ecto

# Get all comments for the given post
Repo.all assoc(post, :comments)

# Or build a query on top of the associated comments
query = from c in assoc(post, :comments), where: not is_nil(c.title)
Repo.all(query)

Another function in Ecto is build_assoc/3, which allows someone to build an associated struct with the proper fields:

Repo.transaction fn ->
  post = Repo.insert!(%Post{title: "Hello", body: "world"})

  # Build a comment from post
  comment = Ecto.build_assoc(post, :comments, body: "Excellent!")

  Repo.insert!(comment)
end

In the example above, Ecto.build_assoc/3 is equivalent to:

%Comment{post_id: post.id, body: "Excellent!"}

You can find more information about defining associations and each respective association module in Ecto.Schema docs.

NOTE: Ecto does not lazy load associations. While lazily loading associations may sound convenient at first, in the long run it becomes a source of confusion and performance issues.

Embeds

Ecto also supports embeds. While associations keep parent and child entries in different tables, embeds stores the child along side the parent.

Databases like Mongo have native support for embeds. Databases like PostgreSQL uses a mixture of JSONB (embeds_one/3) and ARRAY columns to provide this functionality.

Check Ecto.Schema.embeds_one/3 and Ecto.Schema.embeds_many/3 for more information.

Mix tasks and generators

Ecto provides many tasks to help your workflow as well as code generators. You can find all available tasks by typing mix help inside a project with Ecto listed as a dependency.

Ecto generators will automatically open the generated files if you have ECTO_EDITOR set in your environment variable.

Migrations

Ecto supports database migrations. You can generate a migration with:

$ mix ecto.gen.migration create_posts

This will create a new file inside priv/repo/migrations with the up and down functions. Check Ecto.Migration for more information.

Repo resolution

Out of the box, Ecto tasks assumes that the location of your Repo lives within your application’s root namespace; for example, in the previous examples, Ecto will assume your Repo is located at MyApp.Repo.

For more complex use-cases, this might not be sufficient and Ecto allows you to provide an alternative namespace or Repo location using the app_namespace or the app_repo configuration variables as follows:

config :my_app, :app_repo, My.App.Repo
config :my_app, My.App.Repo,
  adapter: Ecto.Adapters.Postgres,
  database: "ecto_simple",
  username: "postgres",
  password: "postgres",
  hostname: "localhost"

In this example the configuration :app_repo is used to explicitly provide the default Repo to use. Alternatively, the :app_namespace config could have been set to My.App to achieve the same result.