Elixir & Phoenix: Scalable Backend Development · Lesson

Repo, Changesets, and Queries

Use `Ecto.Repo` for database interactions, `Changesets` for data validation, and build queries with `Ecto.Query`.

Repo, Changesets, and Queries is a free Elixir & Phoenix: Scalable Backend Development lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Elixir & Phoenix: Scalable Backend Development learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Ecto.Repo: Database Gateway

In Elixir, Ecto.Repo is your primary interface for interacting with your database. Think of it as the central hub that handles all your database operations.

It provides functions for inserting, updating, deleting, and querying records, abstracting away the specifics of the underlying database technology.

Connects your Elixir application to the database.
Manages database transactions.
Executes Ecto queries.

Saving Data with Repo.insert

To add new data to your database, you'll use Ecto.Repo.insert/2. This function takes a changeset (which we'll cover next) and attempts to save it as a new record.

If successful, it returns {:ok, record}; otherwise, it returns {:error, changeset} with validation errors.

defmodule MyApp.Product do
  use Ecto.Schema
  import Ecto.Changeset

  schema "products" do
    field :name, :string
    field :price, :decimal
    field :stock, :integer, default: 0
    timestamps()
  end

  def changeset(product, attrs) do
    product
    |> cast(attrs, [:name, :price, :stock])
    |> validate_required([:name, :price])
    |> validate_number(:price, greater_than: 0)
    |> validate_number(:stock, greater_than_or_equal_to: 0)
  end
end

defmodule MyApp.SimulatedRepo do
  alias MyApp.Product

  def insert(changeset) do
    if Ecto.Changeset.valid?(changeset) do
      data = Ecto.Changeset.apply_action(changeset, :insert)
      # Simulate successful insert with a mock ID and timestamps
      {:ok, %Product{data | id: 1, inserted_at: NaiveDateTime.utc_now(), updated_at: NaiveDateTime.utc_now()}}
    else
      {:error, changeset}
    end
  end
end

defmodule InsertExample do
  alias MyApp.{Product, SimulatedRepo}

  def run() do
    # Prepare product attributes
    attrs = %{name: "Elixir Book", price: Decimal.new("39.99"), stock: 100}

    # Create a changeset
    changeset = Product.changeset(%Product{}, attrs)

    # Simulate inserting the product
    case SimulatedRepo.insert(changeset) do
      {:ok, product} ->
        IO.puts "Successfully inserted product:"
        IO.inspect product
      {:error, changeset} ->
        IO.puts "Failed to insert product:"
        IO.inspect Ecto.Changeset.errors(changeset)
    end
  end
end

# To run this, execute: elixir -e "InsertExample.run()"

Retrieving Records with Repo

You can fetch records from the database using Ecto.Repo.get/3 to retrieve a single record by its primary key, or Ecto.Repo.all/2 to get multiple records.

get(Schema, id): Fetches one record by ID. Returns the struct or nil.
all(Query): Fetches all records matching a given query.

defmodule MyApp.Product do
  use Ecto.Schema
  schema "products" do
    field :name, :string
    field :price, :decimal
    field :stock, :integer, default: 0
    timestamps()
  end
end

defmodule MyApp.SimulatedRepo do
  alias MyApp.Product

  def get(id, _query \\ nil) do
    IO.puts "Simulating fetching product with ID: #{id}"
    case id do
      1 -> %Product{id: 1, name: "Laptop", price: Decimal.new("1200.00"), stock: 5,
                   inserted_at: NaiveDateTime.utc_now(), updated_at: NaiveDateTime.utc_now()}
      _ -> nil
    end
  end

  def all(_query \\ nil) do
    IO.puts "Simulating fetching all products..."
    [
      %Product{id: 1, name: "Laptop", price: Decimal.new("1200.00"), stock: 5,
               inserted_at: NaiveDateTime.utc_now(), updated_at: NaiveDateTime.utc_now()},
      %Product{id: 2, name: "Mouse", price: Decimal.new("25.50"), stock: 20,
               inserted_at: NaiveDateTime.utc_now(), updated_at: NaiveDateTime.utc_now()}
    ]
  end
end

defmodule FetchExample do
  alias MyApp.SimulatedRepo

  def run() do
    # Fetch a single product by ID
    IO.puts "\n--- Fetching by ID ---"
    product = SimulatedRepo.get(1)
    IO.inspect product

    # Fetch all products
    IO.puts "\n--- Fetching all ---"
    products = SimulatedRepo.all()
    IO.inspect products
  end
end

# To run this, execute: elixir -e "FetchExample.run()"

Data Safety with Changesets

Ecto.Changeset is a powerful mechanism for validating, casting, and manipulating data before it interacts with your database. It ensures data integrity and security.

Instead of directly passing raw user input to the database, you first pass it through a changeset. The changeset tracks all proposed changes and any associated errors.

Casting: Converts input data into the correct Elixir types.
Validation: Checks if the data meets predefined rules (e.g., required fields, data formats).
Authorization: Can even be used for field-level permission checks.

Casting and Basic Validation

The first step with a changeset is usually Ecto.Changeset.cast/4. This function takes a struct (or an empty struct), parameters (like user input), and a list of allowed fields. It converts the input into the correct types.

After casting, you can apply validations like validate_required/3 to ensure essential fields are present.

defmodule MyApp.User do
  use Ecto.Schema
  import Ecto.Changeset

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

  def changeset(user, attrs) do
    user
    |> cast(attrs, [:name, :email, :age])
    |> validate_required([:name, :email])
  end
end

defmodule ChangesetCastValidateExample do
  alias MyApp.User

  def run() do
    IO.puts "--- Valid Input ---"
    valid_params = %{"name" => "Alice", "email" => "alice@example.com", "age" => "30"}
    valid_changeset = User.changeset(%User{}, valid_params)
    IO.puts "Is valid? #{Ecto.Changeset.valid?(valid_changeset)}"
    IO.inspect Ecto.Changeset.changes(valid_changeset)

    IO.puts "\n--- Invalid Input (missing email) ---"
    invalid_params = %{"name" => "Bob", "age" => "25"}
    invalid_changeset = User.changeset(%User{}, invalid_params)
    IO.puts "Is valid? #{Ecto.Changeset.valid?(invalid_changeset)}"
    IO.inspect Ecto.Changeset.errors(invalid_changeset)
  end
end

# To run this, execute: elixir -e "ChangesetCastValidateExample.run()"

Deeper Changeset Validations

Beyond basic requirements, Ecto offers many validation helpers to enforce specific rules:

validate_length/3: Checks string length.
validate_number/3: Validates numeric ranges.
validate_format/3: Uses regular expressions for complex patterns (e.g., email format).

These help you build robust data models.

defmodule MyApp.User do
  use Ecto.Schema
  import Ecto.Changeset

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

  def changeset(user, attrs) do
    user
    |> cast(attrs, [:name, :email, :age])
    |> validate_required([:name, :email])
    |> validate_length(:name, min: 3, max: 50)
    |> validate_number(:age, greater_than_or_equal_to: 18)
    |> validate_format(:email, ~r/@/, message: "must contain an @ symbol")
  end
end

defmodule ChangesetDeepValidateExample do
  alias MyApp.User

  def run() do
    IO.puts "--- Valid User ---"
    user_attrs_1 = %{"name" => "Charlie", "email" => "charlie@example.com", "age" => "22"}
    changeset_1 = User.changeset(%User{}, user_attrs_1)
    IO.puts "Valid? #{Ecto.Changeset.valid?(changeset_1)}"
    IO.inspect Ecto.Changeset.errors(changeset_1)

    IO.puts "\n--- Invalid User (short name, young age, bad email) ---"
    user_attrs_2 = %{"name" => "Al", "email" => "al.example.com", "age" => "17"}
    changeset_2 = User.changeset(%User{}, user_attrs_2)
    IO.puts "Valid? #{Ecto.Changeset.valid?(changeset_2)}"
    IO.inspect Ecto.Changeset.errors(changeset_2)
  end
end

# To run this, execute: elixir -e "ChangesetDeepValidateExample.run()"

Applying Changeset Actions

Once a changeset is validated, you can use it to perform database actions like inserting or updating records. The Ecto.Changeset.valid?/1 function tells you if the changeset is ready.

If invalid, Ecto.Changeset.errors/1 will provide a detailed list of what went wrong, which is crucial for providing user feedback.

defmodule MyApp.Product do
  use Ecto.Schema
  import Ecto.Changeset

  schema "products" do
    field :name, :string
    field :price, :decimal
    field :stock, :integer, default: 0
    timestamps()
  end

  def changeset(product, attrs) do
    product
    |> cast(attrs, [:name, :price, :stock])
    |> validate_required([:name, :price])
    |> validate_number(:price, greater_than: 0)
  end
end

defmodule ChangesetApplyExample do
  alias MyApp.Product

  def run() do
    IO.puts "--- Valid Product Changeset ---"
    valid_attrs = %{"name" => "Widget", "price" => "9.99", "stock" => "50"}
    valid_changeset = Product.changeset(%Product{}, valid_attrs)

    if Ecto.Changeset.valid?(valid_changeset) do
      IO.puts "Changeset is valid. Proposed changes:"
      IO.inspect Ecto.Changeset.changes(valid_changeset)
      # In a real app, you'd call Repo.insert(valid_changeset) here
    else
      IO.puts "Changeset is invalid. Errors:"
      IO.inspect Ecto.Changeset.errors(valid_changeset)
    end

    IO.puts "\n--- Invalid Product Changeset (missing price) ---"
    invalid_attrs = %{"name" => "Gadget", "stock" => "20"}
    invalid_changeset = Product.changeset(%Product{}, invalid_attrs)

    if Ecto.Changeset.valid?(invalid_changeset) do
      IO.puts "Changeset is valid. Proposed changes:"
      IO.inspect Ecto.Changeset.changes(invalid_changeset)
    else
      IO.puts "Changeset is invalid. Errors:"
      IO.inspect Ecto.Changeset.errors(invalid_changeset)
    end
  end
end

# To run this, execute: elixir -e "ChangesetApplyExample.run()"

Building Queries with Ecto.Query

While Repo.get/3 and Repo.all/2 are useful for simple fetches, Ecto.Query provides a powerful, composable language for building complex database queries.

You start a query with the from macro, specifying the schema you want to query and giving it an alias.

Ecto queries are Elixir expressions that are translated into SQL by Ecto.

defmodule MyApp.Product do
  use Ecto.Schema
  schema "products" do
    field :name, :string
    field :price, :decimal
    field :stock, :integer, default: 0
  end
end

defmodule EctoQueryFromExample do
  import Ecto.Query
  alias MyApp.Product

  def run() do
    # A simple query to select all products
    query = from p in Product
    IO.puts "Basic query structure:"
    IO.inspect query

    IO.puts "\n(Note: This only shows the query structure, not execution against a DB.)"
  end
end

# To run this, execute: elixir -e "EctoQueryFromExample.run()"

Querying: Filtering & Selecting

You can refine your queries using clauses like where to filter records based on conditions, and select to specify which fields you want to retrieve.

where: p.stock > 0: Filters records where stock is greater than 0.
select: p.name: Retrieves only the 'name' field.

defmodule MyApp.Product do
  use Ecto.Schema
  schema "products" do
    field :name, :string
    field :price, :decimal
    field :stock, :integer, default: 0
  end
end

defmodule EctoQueryWhereSelectExample do
  import Ecto.Query
  alias MyApp.Product

  def run() do
    # Query for products with stock > 0, selecting only their names
    query = from p in Product,
            where: p.stock > 0,
            select: p.name

    IO.puts "Query structure with where and select:"
    IO.inspect query

    IO.puts "\n(Note: This only shows the query structure, not execution against a DB.)"
  end
end

# To run this, execute: elixir -e "EctoQueryWhereSelectExample.run()"

Querying: Ordering & Limiting

To control the presentation of your results, you can use order_by to sort records and limit to restrict the number of records returned.

order_by: [desc: p.price]: Sorts by price in descending order.
limit: 5: Returns only the first 5 matching records.

These are crucial for pagination and displaying sorted lists.

defmodule MyApp.Product do
  use Ecto.Schema
  schema "products" do
    field :name, :string
    field :price, :decimal
    field :stock, :integer, default: 0
  end

end

defmodule EctoQueryOrderLimitExample do
  import Ecto.Query
  alias MyApp.Product

  def run() do
    # Query for products, ordered by price descending, limited to 3
    query = from p in Product,
            order_by: [desc: p.price],
            limit: 3

    IO.puts "Query structure with order_by and limit:"
    IO.inspect query

    IO.puts "\n(Note: This only shows the query structure, not execution against a DB.)"
  end
end

# To run this, execute: elixir -e "EctoQueryOrderLimitExample.run()"

Quick Check: Ecto Core

Which of the following statements about Ecto.Changeset is TRUE?

Recap: Repo, Changesets & Queries

Great job! In this lesson, you've learned the core components for data persistence in Elixir with Ecto:

Ecto.Repo: The gateway for all database interactions.
Ecto.Changeset: Essential for casting, validating, and tracking changes to your data, ensuring integrity.
Ecto.Query: A powerful and flexible way to construct complex database queries using Elixir syntax.

Mastering these three pillars is fundamental to building robust and reliable data-driven applications in Elixir!

Frequently asked questions

Is the “Repo, Changesets, and Queries” lesson free?

Yes — the full text of “Repo, Changesets, and Queries” is free to read here on the web, and the Elixir & Phoenix: Scalable Backend Development course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Elixir & Phoenix: Scalable Backend Development course, upgrade to CoddyKit PRO.

What will I learn in “Repo, Changesets, and Queries”?

Use `Ecto.Repo` for database interactions, `Changesets` for data validation, and build queries with `Ecto.Query`. You practise Elixir & Phoenix: Scalable Backend Development with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Elixir & Phoenix: Scalable Backend Development?

No prior experience is required. Elixir & Phoenix: Scalable Backend Development on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Repo, Changesets, and Queries” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Elixir & Phoenix: Scalable Backend Development lesson?

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