Composable Queries with Ecto Drew Olson @drewolson
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Composable Queries with Ecto Drew Olson
@drewolson
* Brief Ecto Intro * Query Expressions * Composition * Query Pipelines
Please ask questions
Intro
Patterns from real applications
A few models for examples
Intro - Models
defmodule MyApp.Post do use Ecto.Model import Ecto.Query
schema "posts" do field :body, :string field :published, :boolean field :published_at, Ecto.Date field :title, :string
has_many :comments, MyApp.Comment endend
Intro - Models
defmodule MyApp.Comment do use Ecto.Model import Ecto.Query
schema "comments" do field :body, :string field :commenter, :string field :votes, :integer
belongs_to :post, MyApp.Post endend
Keyword query syntax
Intro - Baby’s First Query
posts = MyApp.Repo.all( from p in MyApp.Post, where: p.published == true)
This is how I started writing Ecto queries
Intro
Separate construction and execution
Intro
Intro - First Query Deconstructed
query = from p in MyApp.Post, where: p.published == true
MyApp.Repo.all(query)
Intro - Fancy Query
query = from c in MyApp.Comment, join: p in assoc(c, :post), where: p.id == 1 and c.votes > 5 select: c
comments = MyApp.Repo.all(query)
Query Expressions
Query Expressions - Translation
query = from p in MyApp.Post, where: p.published == true
query = where(MyApp.Post, [p], p.published == true)
Query Expressions - Fancy
query = from c in MyApp.Comment, join: p in assoc(c, :post), where: p.id == 1 and c.votes > 5 select: c
query = MyApp.Comment|> join(:left, [c], p in assoc(c, :post))|> where([_, p], p.id == 1)|> where([c, _], c.votes > 5)|> select([c, _], c)
Composition
Both query styles are composable.
Queries can be the “subject” of new queries.
Composition
Composition
query1 = MyApp.Comment
query2 = from c in query1, where: c.votes > 5
query3 = from c in query2, join: p in assoc(c, :post), where: p.id == 1, select: c
MyApp.Repo.all(query3)
Composition
query1 = MyApp.Comment
query2 = query1|> where([c], c.votes > 5)
query3 = query2|> join(:left, [c], p in assoc(c, :post))|> where([_, p], p.id == 1)|> select([c, _], c)
MyApp.Repo.all(query3)
We can now extract reusable components, name them and compose them.
Composition
Compositiondefmodule MyApp.Comment do ...
def for_post(query, id) do from c in query, join: p in assoc(c, :post), where: p.id == ^id, select: c end
def popular(query) do from c in query, where: c.votes > 5 endend
alias MyApp.Comment
Comment|> Comment.popular|> Comment.for_post(1)|> MyApp.Repo.all
Composition - Reads so nice :)
Comment|> Comment.popular|> Comment.for_post(1)|> MyApp.Repo.all
Query Pipelines
Query Pipelines
Comment|> Comment.popular|> Comment.for_post(1)|> MyApp.Repo.all
Query Pipelines
Comment|> Comment.popular|> Comment.for_post(1)|> MyApp.Repo.all
<- source<- transformation
<- transformation<- sink
Query Pipelines - A note
Welcome to my typespecs, where the types are all made up and the points don’t matter.
Query Pipelines - Source
A source is the starting point for a query.
@spec source() :: Query.t
Query Pipelines - Source Examples
MyApp.Post
MyApp.Post.owned_by(user)
Query Pipelines - Transformation
A transformation expands or constrains an existing query.
@spec transformation(Query.t) :: Query.t
Query Pipelines - Transformation Examples
MyApp.Post.published(query)
MyApp.Comment.for_post(query, post)
Query Pipelines - Sink
A sink executes a query and returns a result.
@spec sink(Query.t) :: Result.t
Query Pipelines - Sink Examples
MyApp.Repo.all(query)
MyApp.Repo.one(query)
MyApp.Repo.paginate(query)
Query Pipelines
Comment|> Comment.popular|> Comment.for_post(1)|> MyApp.Repo.all
Query Pipelines
Pipelines are fractal
Query Pipelines - Fractal
MyApp.Repo.paginate(query)
Query Pipelines
Pagination acts as a sink, but is really several smaller pipelines
defmodule MyApp.Repo do def paginate(query, page_number \\ 1) do {entries(query, page_number), total_entries(query)} end
defp entries(query, page_number) do page_size = 10 offset = page_size * (page_number - 1)
query |> limit([_], ^page_size) |> offset([_], ^offset) |> all end
defp total_entries(query) do query |> exclude(:order_by) |> exclude(:preload) |> exclude(:select) |> select([_], count("*")) |> one endend
Fin
Thanks. Questions?
@drewolson
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