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schema.ex
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defmodule Ecto.Schema do
@moduledoc ~S"""
Defines a schema.
An Ecto schema is used to map any data source into an Elixir struct.
One of such use cases is to map data coming from a repository,
usually a table, into Elixir structs.
## Example
defmodule User do
use Ecto.Schema
schema "users" do
field :name, :string
field :age, :integer, default: 0
has_many :posts, Post
end
end
By default, a schema will automatically generate a primary key which is named
`id` and of type `:integer`. The `field` macro defines a field in the schema
with given name and type. `has_many` associates many posts with the user
schema.
## Schema attributes
Supported attributes, to be set beforehand, for configuring the defined schema.
These attributes are:
* `@primary_key` - configures the schema primary key. It expects
a tuple `{field_name, type, options}` with the primary key field
name, type (typically `:id` or `:binary_id`, but can be any type) and
options. Defaults to `{:id, :id, autogenerate: true}`. When set
to `false`, does not define a primary key in the schema unless
composite keys are defined using the options of `field`.
* `@schema_prefix` - configures the schema prefix. Defaults to `nil`,
which generates structs and queries without prefix. When set, the
prefix will be used by every built struct and on queries where the
current schema is used in `from` (and only `from` exclusively). If
a schema is used as a join or part of an assoc, `@schema_prefix` won't
be obeyed. In PostgreSQL, the prefix is called "SCHEMA" (typically
set via Postgres' `search_path`). In MySQL the prefix points to databases.
* `@foreign_key_type` - configures the default foreign key type
used by `belongs_to` associations. Defaults to `:id`;
* `@timestamps_opts` - configures the default timestamps type
used by `timestamps`. Defaults to `[type: :naive_datetime, usec: true]`;
* `@derive` - the same as `@derive` available in `Kernel.defstruct/1`
as the schema defines a struct behind the scenes;
* `@field_source_mapper` - a function that receives the current field name
and returns the mapping of this field name in the underlying source.
In other words, it is a mechanism to automatically generate the `:source`
option for the `field` macro. It defaults to `fn x -> x end`, where no
field transformation is done;
The advantage of configuring the schema via those attributes is
that they can be set with a macro to configure application wide
defaults.
For example, if your database does not support autoincrementing
primary keys and requires something like UUID or a RecordID, you
can configure and use`:binary_id` as your primary key type as follows:
# Define a module to be used as base
defmodule MyApp.Schema do
defmacro __using__(_) do
quote do
use Ecto.Schema
@primary_key {:id, :binary_id, autogenerate: true}
@foreign_key_type :binary_id
end
end
end
# Now use MyApp.Schema to define new schemas
defmodule MyApp.Comment do
use MyApp.Schema
schema "comments" do
belongs_to :post, MyApp.Post
end
end
Any schemas using `MyApp.Schema` will get the `:id` field with type
`:binary_id` as the primary key. We explain what the `:binary_id` type
entails in the next section.
The `belongs_to` association on `MyApp.Comment` will also define
a `:post_id` field with `:binary_id` type that references the `:id`
field of the `MyApp.Post` schema.
## Primary keys
Ecto supports two ID types, called `:id` and `:binary_id`, which are
often used as the type for primary keys and associations.
The `:id` type is used when the primary key is an integer while the
`:binary_id` is used for primary keys in particular binary formats,
which may be `Ecto.UUID` for databases like PostgreSQL and MySQL,
or some specific ObjectID or RecordID often imposed by NoSQL databases.
In both cases, both types have their semantics specified by the
underlying adapter/database. If you use the `:id` type with
`:autogenerate`, it means the database will be responsible for
auto-generation of the id. This is often the case for primary keys
in relational databases which are auto-incremented.
Similarly, the `:binary_id` type may be generated in the adapter
for cases like UUID but it may also be handled by the database if
required. In any case, both scenarios are handled transparently by
Ecto.
Besides `:id` and `:binary_id`, which are often used by primary
and foreign keys, Ecto provides a huge variety of types to be used
by any column.
Ecto also supports composite primary keys.
## Types and casting
When defining the schema, types need to be given. Types are split
into two categories, primitive types and custom types.
### Primitive types
The primitive types are:
Ecto type | Elixir type | Literal syntax in query
:---------------------- | :---------------------- | :---------------------
`:id` | `integer` | 1, 2, 3
`:binary_id` | `binary` | `<<int, int, int, ...>>`
`:integer` | `integer` | 1, 2, 3
`:float` | `float` | 1.0, 2.0, 3.0
`:boolean` | `boolean` | true, false
`:string` | UTF-8 encoded `string` | "hello"
`:binary` | `binary` | `<<int, int, int, ...>>`
`{:array, inner_type}` | `list` | `[value, value, value, ...]`
`:map` | `map` |
`{:map, inner_type}` | `map` |
`:decimal` | [`Decimal`](https://github.com/ericmj/decimal) |
**Note:** For the `{:array, inner_type}` and `{:map, inner_type}` type,
replace `inner_type` with one of the valid types, such as `:string`.
Since Ecto 2.1, Ecto also supports the Calendar types that are part
of Elixir standard library:
Ecto type | Elixir type
:---------------------- | :----------------------
`:date` | `Date`
`:time` | `Time`
`:naive_datetime` | `NaiveDateTime`
`:utc_datetime` | `DateTime`
Timestamps are typically represented by `:naive_datetime` or
`:utc_datetime`. The naive datetime uses Elixir's `NaiveDateTime` which
has no timezone information while `:utc_datetime` uses a `DateTime` and
expects the time_zone to be set to UTC.
### Custom types
Besides providing primitive types, Ecto allows custom types to be
implemented by developers, allowing Ecto behaviour to be extended.
A custom type is a module that implements the `Ecto.Type` behaviour.
By default, Ecto provides the following custom types:
Custom type | Database type | Elixir type
:---------------------- | :---------------------- | :---------------------
`Ecto.UUID` | `:uuid` | `uuid-string`
Read the `Ecto.Type` documentation for more information on implementing
your own types.
Finally, schemas can also have virtual fields by passing the
`virtual: true` option. These fields are not persisted to the database
and can optionally not be type checked by declaring type `:any`.
### The map type
The map type allows developers to store an Elixir map directly
in the database:
# In your migration
create table(:users) do
add :data, :map
end
# In your schema
field :data, :map
# Now in your code
user = Repo.insert! %User{data: %{"foo" => "bar"}}
Keep in mind that we advise the map keys to be strings or integers
instead of atoms. Atoms may be accepted depending on how maps are
serialized but the database will always return atom keys as strings
due to security reasons.
In order to support maps, different databases may employ different
techniques. For example, PostgreSQL will store those values in jsonb
fields, allowing you to just query parts of it. MySQL and MSSQL, on
the other hand, do not yet provide a JSON type, so the value will be
stored in a text field.
For maps to work in such databases, Ecto will need a JSON library.
By default Ecto will use [Poison](http://github.com/devinus/poison)
which needs to be added your deps in `mix.exs`:
{:poison, "~> 1.0"}
You can however tell Ecto to use any other library by configuring it:
config :ecto, :json_library, YourLibraryOfChoice
If changing the JSON library, remember to recompile Ecto afterwards by
cleaning the current build:
mix deps.clean --build ecto
### Casting
When directly manipulating the struct, it is the responsibility of
the developer to ensure the field values have the proper type. For
example, you can create a user struct with an invalid value
for `age`:
iex> user = %User{age: "0"}
iex> user.age
"0"
However, if you attempt to persist the struct above, an error will
be raised since Ecto validates the types when sending them to the
adapter/database.
Therefore, when working with and manipulating external data, it is
recommended to use `Ecto.Changeset`'s that are able to filter
and properly cast external data:
changeset = Ecto.Changeset.cast(%User{}, %{"age" => "0"}, [:age])
user = Repo.insert!(changeset)
**You can use Ecto schemas and changesets to cast and validate any kind
of data, regardless if the data will be persisted to an Ecto repository
or not**.
## Reflection
Any schema module will generate the `__schema__` function that can be
used for runtime introspection of the schema:
* `__schema__(:source)` - Returns the source as given to `schema/2`;
* `__schema__(:prefix)` - Returns optional prefix for source provided by
`@schema_prefix` schema attribute;
* `__schema__(:primary_key)` - Returns a list of primary key fields (empty if there is none);
* `__schema__(:fields)` - Returns a list of all non-virtual field names;
* `__schema__(:field_source, field)` - Returns the alias of the given field;
* `__schema__(:type, field)` - Returns the type of the given non-virtual field;
* `__schema__(:associations)` - Returns a list of all association field names;
* `__schema__(:association, assoc)` - Returns the association reflection of the given assoc;
* `__schema__(:embeds)` - Returns a list of all embedded field names;
* `__schema__(:embed, embed)` - Returns the embedding reflection of the given embed;
* `__schema__(:read_after_writes)` - Non-virtual fields that must be read back
from the database after every write (insert or update);
* `__schema__(:autogenerate_id)` - Primary key that is auto generated on insert;
Furthermore, both `__struct__` and `__changeset__` functions are
defined so structs and changeset functionalities are available.
"""
@type t :: struct
defmodule Metadata do
@moduledoc """
Stores metadata of a struct.
The fields are:
* `state` - the state in a struct's lifetime, one of `:built`,
`:loaded`, `:deleted`
* `source` - the source for the schema alongside the query prefix,
defaults to `{nil, "source"}`
* `context` - context stored by the database
"""
defstruct [:state, :source, :context]
defimpl Inspect do
import Inspect.Algebra
def inspect(metadata, opts) do
%{source: {prefix, source}, state: state, context: context} = metadata
entries =
for entry <- [state, prefix, source, context],
entry != nil,
do: to_doc(entry, opts)
concat ["#Ecto.Schema.Metadata<"] ++ Enum.intersperse(entries, ", ") ++ [">"]
end
end
end
@doc false
defmacro __using__(_) do
quote do
import Ecto.Schema, only: [schema: 2, embedded_schema: 1]
@primary_key nil
@timestamps_opts []
@foreign_key_type :id
@schema_prefix nil
@field_source_mapper fn x -> x end
Module.register_attribute(__MODULE__, :ecto_primary_keys, accumulate: true)
Module.register_attribute(__MODULE__, :ecto_fields, accumulate: true)
Module.register_attribute(__MODULE__, :ecto_field_sources, accumulate: true)
Module.register_attribute(__MODULE__, :ecto_assocs, accumulate: true)
Module.register_attribute(__MODULE__, :ecto_embeds, accumulate: true)
Module.register_attribute(__MODULE__, :ecto_raw, accumulate: true)
Module.register_attribute(__MODULE__, :ecto_autogenerate, accumulate: true)
Module.register_attribute(__MODULE__, :ecto_autoupdate, accumulate: true)
Module.put_attribute(__MODULE__, :ecto_autogenerate_id, nil)
end
end
@doc """
Defines an embedded schema.
An embedded schema does not require a source name
and it does not include a metadata field.
Embedded schemas by default set the primary key type
to `:binary_id` but such can be configured with the
`@primary_key` attribute.
"""
defmacro embedded_schema([do: block]) do
schema(nil, false, :binary_id, block)
end
@doc """
Defines a schema with a source name and field definitions.
"""
defmacro schema(source, [do: block]) do
schema(source, true, :id, block)
end
defp schema(source, meta?, type, block) do
quote do
@after_compile Ecto.Schema
Module.register_attribute(__MODULE__, :changeset_fields, accumulate: true)
Module.register_attribute(__MODULE__, :struct_fields, accumulate: true)
meta? = unquote(meta?)
source = unquote(source)
prefix = @schema_prefix
# Those module attributes are accessed only dynamically
# so we explicitly reference them here to avoid warnings.
_ = @foreign_key_type
_ = @timestamps_opts
if meta? do
unless is_binary(source) do
raise ArgumentError, "schema source must be a string, got: #{inspect source}"
end
Module.put_attribute(__MODULE__, :struct_fields,
{:__meta__, %Metadata{state: :built, source: {prefix, source}}})
end
if @primary_key == nil do
@primary_key {:id, unquote(type), autogenerate: true}
end
primary_key_fields =
case @primary_key do
false ->
[]
{name, type, opts} ->
Ecto.Schema.__field__(__MODULE__, name, type, [primary_key: true] ++ opts)
[name]
other ->
raise ArgumentError, "@primary_key must be false or {name, type, opts}"
end
try do
import Ecto.Schema
unquote(block)
after
:ok
end
primary_key_fields = @ecto_primary_keys |> Enum.reverse
autogenerate = @ecto_autogenerate |> Enum.reverse
autoupdate = @ecto_autoupdate |> Enum.reverse
fields = @ecto_fields |> Enum.reverse
field_sources = @ecto_field_sources |> Enum.reverse
assocs = @ecto_assocs |> Enum.reverse
embeds = @ecto_embeds |> Enum.reverse
Module.eval_quoted __ENV__, [
Ecto.Schema.__defstruct__(@struct_fields),
Ecto.Schema.__changeset__(@changeset_fields),
Ecto.Schema.__schema__(prefix, source, fields, primary_key_fields),
Ecto.Schema.__types__(fields, field_sources),
Ecto.Schema.__dumper__(fields, field_sources),
Ecto.Schema.__loader__(fields, field_sources),
Ecto.Schema.__field_sources__(fields, field_sources),
Ecto.Schema.__assocs__(assocs),
Ecto.Schema.__embeds__(embeds),
Ecto.Schema.__read_after_writes__(@ecto_raw),
Ecto.Schema.__autogenerate__(@ecto_autogenerate_id, autogenerate, autoupdate)]
end
end
## API
@doc """
Defines a field on the schema with given name and type.
## Options
* `:default` - Sets the default value on the schema and the struct.
The default value is calculated at compilation time, so don't use
expressions like `DateTime.utc_now` or `Ecto.UUID.generate` as
they would then be the same for all records.
* `:source` - Defines the name that is to be used in database for this field.
* `:autogenerate` - Annotates the field to be autogenerated before
insertion if value is not set. It will call the `autogenerate/0`
function in the field's type.
* `:read_after_writes` - When true, the field is always read back
from the database after insert and updates.
For relational databases, this means the RETURNING option of those
statements is used. For this reason, MySQL does not support this
option and will raise an error if a schema is inserted/updated with
read after writes fields.
* `:virtual` - When true, the field is not persisted to the database.
Notice virtual fields do not support `:autogenerate` nor
`:read_after_writes`.
* `:primary_key` - When true, the field is used as part of the
composite primary key
"""
defmacro field(name, type \\ :string, opts \\ []) do
quote do
Ecto.Schema.__field__(__MODULE__, unquote(name), unquote(type), unquote(opts))
end
end
@doc """
Generates `:inserted_at` and `:updated_at` timestamp fields.
The fields generated by this macro will automatically be set to
the current time when inserting and updating values in a repository.
## Options
* `:type` - the timestamps type, defaults to `:naive_datetime`.
* `:usec` - sets whether microseconds are used in timestamps.
Microseconds will be 0 if false. Defaults to true.
* `:inserted_at` - the name of the column for insertion times or `false`
* `:updated_at` - the name of the column for update times or `false`
* `:autogenerate` - a module-function-args tuple used for generating
both `inserted_at` and `updated_at` timestamps
All options can be pre-configured by setting `@timestamps_opts`.
"""
defmacro timestamps(opts \\ []) do
quote bind_quoted: binding() do
timestamps =
[inserted_at: :inserted_at, updated_at: :updated_at,
type: :naive_datetime, usec: true]
|> Keyword.merge(@timestamps_opts)
|> Keyword.merge(opts)
type = Keyword.fetch!(timestamps, :type)
precision = if Keyword.fetch!(timestamps, :usec), do: :microsecond, else: :second
autogen = timestamps[:autogenerate] || {Ecto.Schema, :__timestamps__, [type, precision]}
if inserted_at = Keyword.fetch!(timestamps, :inserted_at) do
Ecto.Schema.field(inserted_at, type, [])
Module.put_attribute(__MODULE__, :ecto_autogenerate, {inserted_at, autogen})
end
if updated_at = Keyword.fetch!(timestamps, :updated_at) do
Ecto.Schema.field(updated_at, type, [])
Module.put_attribute(__MODULE__, :ecto_autogenerate, {updated_at, autogen})
Module.put_attribute(__MODULE__, :ecto_autoupdate, {updated_at, autogen})
end
end
end
@doc ~S"""
Indicates a one-to-many association with another schema.
The current schema has zero or more records of the other schema. The other
schema often has a `belongs_to` field with the reverse association.
## Options
* `:foreign_key` - Sets the foreign key, this should map to a field on the
other schema, defaults to the underscored name of the current schema
suffixed by `_id`
* `:references` - Sets the key on the current schema to be used for the
association, defaults to the primary key on the schema
* `:through` - Allow this association to be defined in terms of existing
associations. Read the section on `:through` associations for more info
* `:on_delete` - The action taken on associations when parent record
is deleted. May be `:nothing` (default), `:nilify_all` and `:delete_all`.
Notice `:on_delete` may also be set in migrations when creating a
reference. If supported, relying on the database via migrations
is preferred. `:nilify_all` and `:delete_all` will not cascade to child
records unless set via database migrations.
* `:on_replace` - The action taken on associations when the record is
replaced when casting or manipulating parent changeset. May be
`:raise` (default), `:mark_as_invalid`, `:nilify`, or `:delete`.
See `Ecto.Changeset`'s section on related data for more info.
* `:defaults` - Default values to use when building the association
## Examples
defmodule Post do
use Ecto.Schema
schema "posts" do
has_many :comments, Comment
end
end
# Get all comments for a given post
post = Repo.get(Post, 42)
comments = Repo.all assoc(post, :comments)
# The comments can come preloaded on the post struct
[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :comments))
post.comments #=> [%Comment{...}, ...]
## has_many/has_one :through
Ecto also supports defining associations in terms of other associations
via the `:through` option. Let's see an example:
defmodule Post do
use Ecto.Schema
schema "posts" do
has_many :comments, Comment
has_one :permalink, Permalink
# In the has_many :through example below, the `:comments`
# in the list [:comments, :author] refers to the
# `has_many :comments` in the Post own schema and the
# `:author` refers to the `belongs_to :author` of the
# Comment's schema (the module below).
# (see the description below for more details)
has_many :comments_authors, through: [:comments, :author]
# Specify the association with custom source
has_many :tags, {"posts_tags", Tag}
end
end
defmodule Comment do
use Ecto.Schema
schema "comments" do
belongs_to :author, Author
belongs_to :post, Post
has_one :post_permalink, through: [:post, :permalink]
end
end
In the example above, we have defined a `has_many :through` association
named `:comments_authors`. A `:through` association always expects a list
and the first element of the list must be a previously defined association
in the current module. For example, `:comments_authors` first points to
`:comments` in the same module (Post), which then points to `:author` in
the next schema, `Comment`.
This `:through` association will return all authors for all comments
that belongs to that post:
# Get all comments for a given post
post = Repo.get(Post, 42)
authors = Repo.all assoc(post, :comments_authors)
Although we used the `:through` association in the example above, Ecto
also allows developers to dynamically build the through associations using
the `Ecto.assoc/2` function:
assoc(post, [:comments, :author])
In fact, given `:through` associations are read-only, **using the `Ecto.assoc/2`
format is the preferred mechanism for working with through associations**. Use
the schema-based one only if you need to store the through data alongside of
the parent struct, in specific cases such as preloading.
`:through` associations can also be preloaded. In such cases, not only
the `:through` association is preloaded but all intermediate steps are
preloaded too:
[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :comments_authors))
post.comments_authors #=> [%Author{...}, ...]
# The comments for each post will be preloaded too
post.comments #=> [%Comment{...}, ...]
# And the author for each comment too
hd(post.comments).author #=> %Author{...}
When the `:through` association is expected to return one or zero items,
`has_one :through` should be used instead, as in the example at the beginning
of this section:
# How we defined the association above
has_one :post_permalink, through: [:post, :permalink]
# Get a preloaded comment
[comment] = Repo.all(Comment) |> Repo.preload(:post_permalink)
comment.post_permalink #=> %Permalink{...}
"""
defmacro has_many(name, queryable, opts \\ []) do
queryable = expand_alias(queryable, __CALLER__)
quote do
Ecto.Schema.__has_many__(__MODULE__, unquote(name), unquote(queryable), unquote(opts))
end
end
@doc ~S"""
Indicates a one-to-one association with another schema.
The current schema has zero or one records of the other schema. The other
schema often has a `belongs_to` field with the reverse association.
## Options
* `:foreign_key` - Sets the foreign key, this should map to a field on the
other schema, defaults to the underscored name of the current schema
suffixed by `_id`
* `:references` - Sets the key on the current schema to be used for the
association, defaults to the primary key on the schema
* `:through` - If this association must be defined in terms of existing
associations. Read the section in `has_many/3` for more information
* `:on_delete` - The action taken on associations when parent record
is deleted. May be `:nothing` (default), `:nilify_all` and `:delete_all`.
Notice `:on_delete` may also be set in migrations when creating a
reference. If supported, relying on the database via migrations
is preferred. `:nilify_all` and `:delete_all` will not cascade to child
records unless set via database migrations.
* `:on_replace` - The action taken on associations when the record is
replaced when casting or manipulating parent changeset. May be
`:raise` (default), `:mark_as_invalid`, `:nilify`, `:update`, or
`:delete`. See `Ecto.Changeset`'s section on related data for more info.
* `:defaults` - Default values to use when building the association
## Examples
defmodule Post do
use Ecto.Schema
schema "posts" do
has_one :permalink, Permalink
# Specify the association with custom source
has_one :category, {"posts_categories", Category}
end
end
# The permalink can come preloaded on the post struct
[post] = Repo.all(from(p in Post, where: p.id == 42, preload: :permalink))
post.permalink #=> %Permalink{...}
"""
defmacro has_one(name, queryable, opts \\ []) do
queryable = expand_alias(queryable, __CALLER__)
quote do
Ecto.Schema.__has_one__(__MODULE__, unquote(name), unquote(queryable), unquote(opts))
end
end
@doc ~S"""
Indicates a one-to-one or many-to-one association with another schema.
The current schema belongs to zero or one records of the other schema. The other
schema often has a `has_one` or a `has_many` field with the reverse association.
You should use `belongs_to` in the table that contains the foreign key. Imagine
a company <-> employee relationship. If the employee contains the `company_id` in
the underlying database table, we say the employee belongs to company.
In fact, when you invoke this macro, a field with the name of foreign key is
automatically defined in the schema for you.
## Options
* `:foreign_key` - Sets the foreign key field name, defaults to the name
of the association suffixed by `_id`. For example, `belongs_to :company`
will define foreign key of `:company_id`
* `:references` - Sets the key on the other schema to be used for the
association, defaults to: `:id`
* `:define_field` - When false, does not automatically define a `:foreign_key`
field, implying the user is defining the field manually elsewhere
* `:type` - Sets the type of automatically defined `:foreign_key`.
Defaults to: `:integer` and can be set per schema via `@foreign_key_type`
* `:on_replace` - The action taken on associations when the record is
replaced when casting or manipulating parent changeset. May be
`:raise` (default), `:mark_as_invalid`, `:nilify`, `:update`, or `:delete`.
See `Ecto.Changeset`'s section on related data for more info.
* `:defaults` - Default values to use when building the association
* `:primary_key` - If the underlying belongs_to field is a primary key
* `:source` - The source for the underlying field
## Examples
defmodule Comment do
use Ecto.Schema
schema "comments" do
belongs_to :post, Post
end
end
# The post can come preloaded on the comment record
[comment] = Repo.all(from(c in Comment, where: c.id == 42, preload: :post))
comment.post #=> %Post{...}
If you need custom options on the underlying field, you can define the
field explicitly and then pass `define_field: false` to `belongs_to`:
defmodule Comment do
use Ecto.Schema
schema "comments" do
field :post_id, :integer, ... # custom options
belongs_to :post, Post, define_field: false
end
end
## Polymorphic associations
One common use case for belongs to associations is to handle
polymorphism. For example, imagine you have defined a Comment
schema and you wish to use it for commenting on both tasks and
posts.
Some abstractions would force you to define some sort of
polymorphic association with two fields in your database:
* commentable_type
* commentable_id
The problem with this approach is that it breaks references in
the database. You can't use foreign keys and it is very inefficient,
both in terms of query time and storage.
In Ecto, we have three ways to solve this issue. The simplest
is to define multiple fields in the Comment schema, one for each
association:
* task_id
* post_id
Unless you have dozens of columns, this is simpler for the developer,
more DB friendly and more efficient in all aspects.
Alternatively, because Ecto does not tie a schema to a given table,
we can use separate tables for each association. Let's start over
and define a new Comment schema:
defmodule Comment do
use Ecto.Schema
schema "abstract table: comments" do
# This will be used by associations on each "concrete" table
field :assoc_id, :integer
end
end
Notice we have changed the table name to "abstract table: comments".
You can choose whatever name you want, the point here is that this
particular table will never exist.
Now in your Post and Task schemas:
defmodule Post do
use Ecto.Schema
schema "posts" do
has_many :comments, {"posts_comments", Comment}, foreign_key: :assoc_id
end
end
defmodule Task do
use Ecto.Schema
schema "tasks" do
has_many :comments, {"tasks_comments", Comment}, foreign_key: :assoc_id
end
end
Now each association uses its own specific table, "posts_comments"
and "tasks_comments", which must be created on migrations. The
advantage of this approach is that we never store unrelated data
together, also ensuring we keep database references fast and correct.
When using this technique, the only limitation is that you cannot
build comments directly. For example, the command below
Repo.insert!(%Comment{})
will attempt to use the abstract table. Instead, one should use
Repo.insert!(build_assoc(post, :comments))
where `build_assoc/3` is defined in `Ecto`. You can also
use `assoc/2` in both `Ecto` and in the query syntax
to easily retrieve associated comments to a given post or
task:
# Fetch all comments associated with the given task
Repo.all(assoc(task, :comments))
Or all comments in a given table:
Repo.all from(c in {"posts_comments", Comment}), ...)
The third and final option is to use `many_to_many/3` to
define the relationships between the resources. In this case,
the comments table won't have the foreign key, instead there
is a intermediary table responsible for associating the entries:
defmodule Comment do
use Ecto.Schema
schema "comments" do
# ...
end
end
In your posts and tasks:
defmodule Post do
use Ecto.Schema
schema "posts" do
many_to_many :comments, Comment, join_through: "posts_comments"
end
end
defmodule Task do
use Ecto.Schema
schema "tasks" do
many_to_many :comments, Comment, join_through: "tasks_comments"
end
end
See `many_to_many/3` for more information on this particular approach.
"""
defmacro belongs_to(name, queryable, opts \\ []) do
queryable = expand_alias(queryable, __CALLER__)
quote do
Ecto.Schema.__belongs_to__(__MODULE__, unquote(name), unquote(queryable), unquote(opts))
end
end
@doc ~S"""
Indicates a many-to-many association with another schema.
The association happens through a join schema or source, containing
foreign keys to the associated schemas. For example, the association
below:
# from MyApp.Post
many_to_many :tags, MyApp.Tag, join_through: "posts_tags"
is backed by relational databases through a join table as follows:
[Post] <-> [posts_tags] <-> [Tag]
id <-- post_id
tag_id --> id
More information on the migration for creating such a schema is shown
below.
## Options
* `:join_through` - specifies the source of the associated data.
It may be a string, like "posts_tags", representing the
underlying storage table or an atom, like `MyApp.PostTag`,
representing a schema. This option is required.
* `:join_keys` - specifies how the schemas are associated. It
expects a keyword list with two entries, the first being how
the join table should reach the current schema and the second
how the join table should reach the associated schema. In the
example above, it defaults to: `[post_id: :id, tag_id: :id]`.
The keys are inflected from the schema names.
* `:on_delete` - The action taken on associations when the parent record
is deleted. May be `:nothing` (default) or `:delete_all`.
`:delete_all` will only remove data from the join source, never the
associated records. Notice `:on_delete` may also be set in migrations
when creating a reference. If supported, relying on the database via
migrations is preferred. `:nilify_all` and `:delete_all` will not cascade
to child records unless set via database migrations.
* `:on_replace` - The action taken on associations when the record is
replaced when casting or manipulating parent changeset. May be
`:raise` (default), `:mark_as_invalid`, or `:delete`.
`:delete` will only remove data from the join source, never the
associated records. See `Ecto.Changeset`'s section on related data
for more info.
* `:defaults` - Default values to use when building the association
* `:unique` - When true, checks if the associated entries are unique.
This is done by checking the primary key of the associated entries during
repository operations. Keep in mind this does not guarantee uniqueness at the
database level. For such it is preferred to set a unique index in the database.
For example: `create unique_index(:posts_tags, [:post_id, :tag_id])`
## Removing data
If you attempt to remove associated `many_to_many` data, **Ecto will
always remove data from the join schema and never from the target
associations** be it by setting `:on_replace` to `:delete`, `:on_delete`
to `:delete_all` or by using changeset functions such as
`Ecto.Changeset.put_assoc/3`. For example, if a `Post` has a many to many
relationship with `Tag`, setting `:on_delete` to `:delete_all` will
only delete entries from the "posts_tags" table in case `Post` is
deleted.
## Migration
How your migration should be structured depends on the value you pass
in `:join_through`. If `:join_through` is simply a string, representing
a table, you may define a table without primary keys and you must not
include any further columns, as those values won't be set by Ecto:
create table(:posts_tags, primary_key: false) do
add :post_id, references(:posts)
add :tag_id, references(:tags)
end
However, if your `:join_through` is a schema, like `MyApp.PostTag`, your
join table may be structured as any other table in your codebase,
including timestamps:
create table(:posts_tags) do
add :post_id, references(:posts)
add :tag_id, references(:tags)
timestamps
end
Because `:join_through` contains a schema, in such cases, autogenerated
values and primary keys will be automatically handled by Ecto.
## Examples
defmodule Post do
use Ecto.Schema
schema "posts" do
many_to_many :tags, Tag, join_through: "posts_tags"
end
end