# `Ecto`
[🔗](https://github.com/elixir-ecto/ecto/blob/v3.13.6/lib/ecto.ex#L1)

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 are used to map external data into Elixir
    structs. We often use them to map database tables to Elixir data but
    they have many other use cases

  * `Ecto.Query` - written in Elixir syntax, queries are used to retrieve
    information from a given repository. Ecto queries are secure and composable

  * `Ecto.Changeset` - changesets provide a way to track and validate changes
    before they are applied to the data

In summary:

  * `Ecto.Repo` - **where** the data is
  * `Ecto.Schema` - **what** the data is
  * `Ecto.Query` - **how to read** the data
  * `Ecto.Changeset` - **how to change** the data

Besides the four components above, most developers use Ecto to interact
with SQL databases, such as PostgreSQL and MySQL via the
[`ecto_sql`](https://hexdocs.pm/ecto_sql) project. `ecto_sql` provides many
conveniences for working with SQL databases as well as the ability to version
how your database changes through time via
[database migrations](https://hexdocs.pm/ecto_sql/Ecto.Adapters.SQL.html#module-migrations).

If you want to quickly check a sample application using Ecto, please check
the [getting started guide](https://hexdocs.pm/ecto/getting-started.html) and
the accompanying sample application. [Ecto's README](https://github.com/elixir-ecto/ecto)
also links to other resources.

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.

## 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,
        adapter: Ecto.Adapters.Postgres
    end

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

    config :my_app, Repo,
      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 is 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/application.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
      children = [
        MyApp.Repo,
      ]

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

## Schema

Schemas allow developers to define the shape of their data.
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 schema. 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;

  * You do not need to define schemas in order to interact with repositories,
    operations like `all`, `insert_all` and so on allow developers to directly
    access and modify the data, keeping the database at your fingertips when
    necessary;

## Changesets

Although in the example above we have directly inserted and deleted the
struct in the repository, operations on top of schemas are done through
changesets so Ecto can efficiently track changes.

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/4` with
the struct, the parameters and a list of allowed fields; this returns a changeset.
The parameters is a map with binary keys and values that will be cast based
on the type defined by the schema.

Any parameter that was not explicitly listed in the fields list will be ignored.

After casting, the changeset is given to many `Ecto.Changeset.validate_*`
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.

Once a changeset is built, it can be given to functions like `insert` and
`update` in the repository that will return an `:ok` or `:error` tuple:

    case Repo.update(changeset) do
      {:ok, user} ->
        # user updated
      {:error, changeset} ->
        # an error occurred
    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 registering and updating
users:

    def registration_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 u in User,
              where: u.age > 18 or is_nil(u.email),
              select: u

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

In the example above we relied on our schema but queries can also be
made directly against a table by giving the table name as a string. In
such cases, the data to be fetched must be explicitly outlined:

    query = from u in "users",
              where: u.age > 18 or is_nil(u.email),
              select: %{name: u.name, age: u.age}

    # Returns maps as defined in select
    Repo.all(query)

Queries are defined and extended with the [`from`](`Ecto.Query.from/2`) 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_age(min) do
      from u in User, where: u.age > ^min
    end

Besides [`Repo.all/1`](`c:Ecto.Repo.all/2`) which returns all entries, repositories
also provide [`Repo.one/1`](`c:Ecto.Repo.one/2`) which returns one entry or `nil`,
[`Repo.one!/1`](`c:Ecto.Repo.one!/2`)) which returns one entry or raises,
[`Repo.get/2`](`c:Ecto.Repo.get/3`) which fetches entries for a particular ID and more.

Finally, if you need an 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/3` 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.transact(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 MongoDB 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.

#### Repo resolution

Ecto requires developers to specify the key `:ecto_repos` in their
application configuration before using tasks like `ecto.create` and
`ecto.migrate`. For example:

    config :my_app, :ecto_repos, [MyApp.Repo]

    config :my_app, MyApp.Repo,
      database: "ecto_simple",
      username: "postgres",
      password: "postgres",
      hostname: "localhost"

# `assoc`

Builds a query for the association in the given struct or structs.

## Examples

In the example below, we get all comments associated to the given
post:

    post = Repo.get Post, 1
    Repo.all Ecto.assoc(post, :comments)

`assoc/3` can also receive a list of posts, as long as the posts are
not empty:

    posts = Repo.all from p in Post, where: is_nil(p.published_at)
    Repo.all Ecto.assoc(posts, :comments)

This function can also be used to dynamically load through associations
by giving it a list. For example, to get all authors for all comments for
the given posts, do:

    posts = Repo.all from p in Post, where: is_nil(p.published_at)
    Repo.all Ecto.assoc(posts, [:comments, :author])

## Options

  * `:prefix` - the prefix to fetch assocs from. By default, queries
    will use the same prefix as the first struct in the given collection.
    This option allows the prefix to be changed.

# `assoc_loaded?`

Checks if an association is loaded.

## Examples

    iex> post = Repo.get(Post, 1)
    iex> Ecto.assoc_loaded?(post.comments)
    false
    iex> post = post |> Repo.preload(:comments)
    iex> Ecto.assoc_loaded?(post.comments)
    true

# `build_assoc`

Builds a struct from the given `assoc` in `struct`.

## Examples

If the relationship is a `has_one` or `has_many` and
the primary key is set in the parent struct, the key will
automatically be set in the built association:

    iex> post = Repo.get(Post, 13)
    %Post{id: 13}
    iex> build_assoc(post, :comments)
    %Comment{id: nil, post_id: 13}

Note though it doesn't happen with `belongs_to` cases, as the
key is often the primary key and such is usually generated
dynamically:

    iex> comment = Repo.get(Comment, 13)
    %Comment{id: 13, post_id: 25}
    iex> build_assoc(comment, :post)
    %Post{id: nil}

You can also pass the attributes, which can be a map or
a keyword list, to set the struct's fields except the
association key.

    iex> build_assoc(post, :comments, text: "cool")
    %Comment{id: nil, post_id: 13, text: "cool"}

    iex> build_assoc(post, :comments, %{text: "cool"})
    %Comment{id: nil, post_id: 13, text: "cool"}

    iex> build_assoc(post, :comments, post_id: 1)
    %Comment{id: nil, post_id: 13}

The given attributes are expected to be structured data.
If you want to build an association with external data,
such as a request parameters, you can use `Ecto.Changeset.cast/3`
after `build_assoc/3`:

    parent
    |> Ecto.build_assoc(:child)
    |> Ecto.Changeset.cast(params, [:field1, :field2])

# `embedded_dump`

```elixir
@spec embedded_dump(Ecto.Schema.t(), format :: atom()) :: map()
```

Dumps the given struct defined by an embedded schema.

This converts the given embedded schema to a map to be serialized
with the given format. For example:

    iex> Ecto.embedded_dump(%Post{}, :json)
    %{title: "hello"}

# `embedded_load`

```elixir
@spec embedded_load(
  module_or_map :: module() | map(),
  data :: map(),
  format :: atom()
) :: Ecto.Schema.t() | map()
```

Loads previously dumped `data` in the given `format` into a schema.

The first argument can be an embedded schema module, or a map (of types) and
determines the return value: a struct or a map, respectively.

The second argument `data` specifies fields and values that are to be loaded.
It can be a map, a keyword list, or a `{fields, values}` tuple. Fields can be
atoms or strings.

The third argument `format` is the format the data has been dumped as. For
example, databases may dump embedded to `:json`, this function allows such
dumped data to be put back into the schemas. If custom types are used,
Ecto will invoke the `c:Ecto.Type.embed_as/1` callback to decide if the data
should be loaded using `cast` or `load`.

Fields that are not present in the schema (or `types` map) are ignored.
If any of the values has invalid type, an error is raised.

Note that if you want to load data into a non-embedded schema that was
directly persisted into a given repository, then use `c:Ecto.Repo.load/2`.

## Examples

    iex> result = Ecto.Adapters.SQL.query!(MyRepo, "SELECT users.settings FROM users", [])
    iex> Enum.map(result.rows, fn [settings] -> Ecto.embedded_load(Setting, Jason.decode!(settings), :json) end)
    [%Setting{...}, ...]

# `get_meta`

Gets the metadata from the given struct.

For example, to check whether it has been persisted:

    iex> Ecto.get_meta(changeset.data, :state)
    :built

See `Ecto.Schema.Metadata`.

# `primary_key`

```elixir
@spec primary_key(Ecto.Schema.t()) :: Keyword.t()
```

Returns the schema primary keys as a keyword list.

# `primary_key!`

```elixir
@spec primary_key!(Ecto.Schema.t()) :: Keyword.t()
```

Returns the schema primary keys as a keyword list.

Raises `Ecto.NoPrimaryKeyFieldError` if the schema has no
primary key field.

# `put_meta`

```elixir
@spec put_meta(Ecto.Schema.schema(), meta) :: Ecto.Schema.schema()
when meta: [
       source: Ecto.Schema.source(),
       prefix: Ecto.Schema.prefix(),
       context: Ecto.Schema.Metadata.context(),
       state: Ecto.Schema.Metadata.state()
     ]
```

Returns a new struct with updated metadata.

It is possible to set:

  * `:source` - changes the struct query source
  * `:prefix` - changes the struct query prefix
  * `:context` - changes the struct meta context
  * `:state` - changes the struct state

See `Ecto.Schema.Metadata`.

# `reset_fields`

```elixir
@spec reset_fields(Ecto.Schema.t(), list()) :: Ecto.Schema.t()
```

Resets fields in a struct to their default values.

## Examples

    iex> post = post |> Repo.preload(:author)
    %Post{title: "hello world", author: %Author{}}
    iex> Ecto.reset_fields(post, [:title, :author])
    %Post{
      title: "default title",
      author: #Ecto.Association.NotLoaded<association :author is not loaded>
    }

---

*Consult [api-reference.md](api-reference.md) for complete listing*