Query Builder

QueryBuilder is a thin layer over Ecto that builds composable queries from plain Elixir data structures.

Index

• Key Features
• Setup
• Feature Overview
• Examples

Key Features

Controller/Resolver‑Driven Query Options

Controllers/GraphQL resolvers can pass filter/sort/page options into a single context list function via from_opts/2, without creating new context functions per option or writing custom option-handling logic in the context.

def list_users(opts \\ []) do
  User
  |> QueryBuilder.where(deleted: false)
  |> QueryBuilder.from_opts(opts)
  |> Repo.all()
end

# controller/resolver
list_users(where: [name: "Alice"], order_by: [desc: :inserted_at], limit: 50)

For optional params, use maybe_where/* / maybe_order_by/* to conditionally apply clauses.

def list_users(opts \\ []) do
  include_deleted? = Keyword.get(opts, :include_deleted?, false)
  qb_opts = Keyword.drop(opts, [:include_deleted?])

  User
  |> QueryBuilder.maybe_where(not include_deleted?, deleted: false)
  |> QueryBuilder.maybe_order_by(not Keyword.has_key?(qb_opts, :order_by), desc: :inserted_at, desc: :id)
  |> QueryBuilder.from_opts(qb_opts)
  |> Repo.all()
end

Data‑Driven Query Composition

QueryBuilder lets you express complex filtering and composition as plain Elixir data, without positional binding gymnastics and without manually building dynamic/2 trees for the common cases.

QueryBuilder lets you express “OR of AND groups” directly as nested lists:

# (name == "Alice") OR (name == "Bob" AND deleted == false)
or_groups = [[name: "Alice"], [name: "Bob", deleted: false]]

User
|> QueryBuilder.where(active: true)
|> QueryBuilder.where_any(or_groups)
|> Repo.all()

In Ecto, when the OR groups come from runtime data (e.g. controller params), you typically have to reduce them into a dynamic/2 expression:

# (name == "Alice") OR (name == "Bob" AND deleted == false)
or_groups = [[name: "Alice"], [name: "Bob", deleted: false]]

or_dynamic =
  Enum.reduce(or_groups, dynamic([u], false), fn group, or_acc ->
    and_dynamic =
      Enum.reduce(group, dynamic([u], true), fn {field, value}, and_acc ->
        dynamic([u], ^and_acc and field(u, ^field) == ^value)
      end)

    dynamic([u], ^or_acc or ^and_dynamic)
  end)

User
|> where([u], u.active == true)
|> where(^or_dynamic)

Assoc Queries Without Binding Boilerplate

QueryBuilder lets you reference association fields with @ tokens (e.g. :name@role) instead of manually writing joins and positional binding lists.

User
|> QueryBuilder.order_by(:role, asc: :name@role, asc: :nickname)
|> Repo.all()

Ecto:

User
|> join(:left, [u], r in assoc(u, :role))
|> order_by([u, r], asc: r.name, asc: u.nickname)
|> Repo.all()

Rich Filter DSL (Operators + Field Comparisons)

Beyond {field, value} equality, you can use {field, operator, value} for common operators (ranges, membership, text search). For field-to-field comparisons, use the @self marker as the value.

nickname_query = "admin"

filters = [
  {:nickname, :contains, nickname_query, [case: :i]},
  {:inserted_at, :ge, from},
  {:id, :in, ids}
]

User
|> QueryBuilder.where(filters)
|> Repo.all()

User
|> QueryBuilder.where_exists_subquery([authored_articles: :comments],
  scope: [],
  where: [
    {:body@comments, :contains, :nickname@self, [case: :insensitive]}
  ]
)
|> Repo.all()

Keyset/Cursor-Based Pagination

paginate/3 returns an opaque cursor derived from your order_by; pass it back unchanged to fetch the next/previous page.

# First page (no cursor)
pagination_opts = [page_size: 10]

%{paginated_entries: users, pagination: page} =
  User
  |> QueryBuilder.order_by(asc: :nickname, desc: :email)
  |> QueryBuilder.paginate(Repo, pagination_opts)

# Next page: pass back the opaque cursor returned in pagination
pagination_opts =
  Keyword.merge(pagination_opts,
    cursor: page.cursor_for_entries_after,
    direction: :after
  )

%{paginated_entries: next_users, pagination: next_page} =
  User
  |> QueryBuilder.order_by(asc: :nickname, desc: :email)
  |> QueryBuilder.paginate(Repo, pagination_opts)

Higher‑Level Query Helpers

QueryBuilder also includes higher-level helpers that are verbose to write correctly in raw Ecto.

alias QueryBuilder, as: QB

# Latest child row per parent
User
|> QB.left_join_latest(:authored_articles, order_by: [desc: :inserted_at, desc: :id])
|> Repo.all()
# => [{%User{}, %Article{} | nil}, ...]

# Top N rows per group
Post
|> QB.top_n_per(partition_by: [:subreddit_id], order_by: [desc: :score, desc: :id], n: 3)
|> Repo.all()

Custom, User-Defined Query Operations (Extension)

QueryBuilder.Extension lets you build an app-specific “QB module” that adds your own query operations on top of QueryBuilder.

defmodule MyApp.QB do
  use QueryBuilder.Extension, from_opts_full_ops: [:where_initcap]
  import Ecto.Query

  def where_initcap(query, field, value) do
    where(query, fn resolve ->
      {field, binding} = resolve.(field)
      dynamic([{^binding, x}], fragment("initcap(?)", field(x, ^field)) == ^value)
    end)
  end
end

# trusted/internal (full mode)
alias MyApp.QB

MyApp.User
|> QB.where_initcap(:name, "Alice")
|> Repo.all()

Setup

Add query_builder as a dependency:

def deps do
  [
    {:query_builder, "~> 2.0.0"}
  ]
end

Feature Overview

Operations

• Filtering: where/*, where_any/*, maybe_where/*
• Sorting: order_by/*, maybe_order_by/*
• Offset pagination: limit/2, offset/2
• Keyset pagination: paginate/3 (cursor-based)
• Joins: inner_join/2, left_join/4, left_join_leaf/4, left_join_path/4
• To-many existence filters: where_exists_subquery/3, where_not_exists_subquery/3, where_has/3, where_missing/3
• Preloads: preload_separate/2, preload_separate_scoped/3, preload_through_join/2
• Selection & distinctness: select/*, select_merge/*, distinct/*, distinct_roots/1 (Postgres-only)
• Grouping & aggregates: group_by/*, having/*, having_any/*, aggregates (count/*, count_distinct/1, avg/1, sum/1, min/1, max/1, array_agg/* (Postgres-only))
• Postgres query patterns: top_n_per/*, first_per/*, left_join_latest/3, left_join_top_n/3

Tokens, assoc paths, and join intent

• Tokens are atoms/strings: :field, :field@assoc, or full paths like :field@assoc@nested_assoc....
• field@assoc is shorthand and raises if @assoc is ambiguous; use a full-path token to disambiguate.
• Assoc paths (assoc_fields) support join markers:
  • :role (neutral): reuse an existing join qualifier if already joined; otherwise QueryBuilder defaults to LEFT
  • :role?: force LEFT
  • :role!: force INNER
• @self marks field-to-field comparisons (e.g. {:inserted_at@comments, :gt, :inserted_at@self}).

from_opts

• from_opts/2 defaults to boundary mode (for controllers/resolvers): allowlists where, where_any, order_by, limit, offset.
• from_opts/3 with mode: :full enables the full QueryBuilder surface (use when the caller knows the base query’s implementation/shape).
• args/* wraps multiple arguments for from_opts(..., mode: :full) (e.g. calling where/4, order_by/3, select/3, or extension ops).

Extensions

• QueryBuilder.Extension lets you define an app-specific module that wraps QueryBuilder and adds custom operations you can call directly.
• If you use from_opts on that module, you must explicitly allowlist which custom operations are callable via from_opts_full_ops: [...] (full mode) and optionally boundary_ops_user_asserted: [...] (boundary mode).

Utilities

• new/1: wrap an existing Ecto queryable into a %QueryBuilder.Query{}.
• subquery/2: build an Ecto.SubQuery using QueryBuilder operations (from_opts(..., mode: :full) + Ecto.Query.subquery/1).
• default_page_size/0: reads config :query_builder, :default_page_size.

Examples

Filter “has related rows” (to-many) without duplicates

Filter root rows through a to-many association via correlated EXISTS(...) without join-multiplying roots.

alias QueryBuilder, as: QB

User
|> QB.where_has(:authored_articles, published@authored_articles: true)
|> Repo.all()

Filter “missing related rows” (to-many)

Filter root rows through a to-many association via correlated NOT EXISTS(...).

alias QueryBuilder, as: QB

User
|> QB.where_missing(:authored_articles)
|> Repo.all()

Ensure unique roots after joining a to-many association (Postgres)

When you must join a to-many association and still want unique root rows (especially with limit/offset), use distinct_roots/1.

alias QueryBuilder, as: QB

User
|> QB.left_join(:authored_articles)
|> QB.order_by(asc: :id)
|> QB.order_by(:authored_articles, desc: :inserted_at@authored_articles, desc: :id@authored_articles)
|> QB.distinct_roots()
|> QB.offset(20)
|> QB.limit(10)
|> Repo.all()

Scoped separate preload (Ecto query-preload equivalent)

Preload a direct association with an explicit scope using a separate query (preload_separate_scoped/3).

alias QueryBuilder, as: QB

User
|> QB.preload_separate_scoped(:authored_articles,
  where: [published: true],
  order_by: [desc: :inserted_at]
)
|> Repo.all()

Join-scoped preload (preload only joined rows)

Preload an association through its join binding so preloaded rows reflect the join (including join on: filters).

alias QueryBuilder, as: QB

User
|> QB.left_join(:authored_articles, published@authored_articles: true)
|> QB.preload_through_join(:authored_articles)
|> Repo.all()

Nested join semantics: LEFT every hop vs INNER path + LEFT leaf

Choose whether intermediate hops in a nested path are INNER (left_join_leaf/4) or LEFT (left_join_path/4).

alias QueryBuilder, as: QB

# INNER authored_articles, LEFT comments
q1 = User |> QB.left_join_leaf([authored_articles: :comments])

# LEFT authored_articles, LEFT comments
q2 = User |> QB.left_join_path([authored_articles: :comments])

Grouping + HAVING with aggregate helpers

Group and filter groups using group_by/* + having/* with aggregate helpers like count/0.

alias QueryBuilder, as: QB

User
|> QB.group_by(:role, :name@role)
|> QB.having([{QB.count(:id), :gt, 10}])
|> QB.select({:name@role, QB.count(:id)})
|> Repo.all()

array_agg with DISTINCT, ORDER BY, and FILTER (Postgres)

Build grouped results with Postgres aggregates like array_agg (including FILTER (WHERE ...)).

alias QueryBuilder, as: QB

Article
|> QB.group_by(:author_id)
|> QB.select(%{
  author_id: :author_id,
  publisher_ids:
    QB.array_agg(:publisher_id,
      distinct?: true,
      order_by: [asc: :publisher_id],
      filter: [{:publisher_id, :ne, nil}]
    )
})
|> Repo.all()

Build an IN (subquery) using QueryBuilder ops

Use subquery/2 to build an Ecto.SubQuery from QueryBuilder options and use it in filters.

alias QueryBuilder, as: QB

active_user_ids =
  QB.subquery(User,
    where: [active: true],
    select: :id
  )

Article
|> QB.where({:author_id, :in, active_user_ids})
|> Repo.all()

Top N children per parent (Postgres, LATERAL)

Fetch up to N association rows per parent via LEFT JOIN LATERAL and group {parent, child} rows in Elixir.

alias QueryBuilder, as: QB

rows =
  User
  |> QB.left_join_top_n(:authored_articles, n: 3, order_by: [desc: :inserted_at, desc: :id])
  |> Repo.all()

top_articles_by_user_id =
  Enum.group_by(rows, fn {u, _a} -> u.id end, fn {_u, a} -> a end)