Runtime Lookups

While belongs_to syncs a fixture and uses the result, sometimes you need to reference data that already exists in the database without syncing it. That's where Sow.lookup comes in.

Overview

Basic Usage

defmodule MyApp.Seeds.Organizations do
  use Sow, schema: MyApp.Organization, keys: [:slug]

  def records do
    [
      %{
        slug: "acme-norway",
        name: "ACME Norway",
        # Query the database directly for country.id where code = "NO"
        country_id: Sow.lookup(MyApp.Country, :code, "NO")
      }
    ]
  end
end

When to Use Lookup

1. Referencing Core Data

When your fixtures reference data managed elsewhere (migrations, admin UI, etc.):

# Countries are managed in migrations, not fixtures
country_id: Sow.lookup(MyApp.Country, :code, "NO")

2. Cross-Module References

When fixtures in different modules reference each other without creating dependencies:

# Don't want to sync Users fixture, just reference existing user
created_by_id: Sow.lookup(MyApp.User, :email, "admin@example.com")

3. Avoiding Circular Dependencies

When two fixtures would otherwise create a cycle:

# Instead of belongs_to which would create dependency
parent_org_id: Sow.lookup(MyApp.Organization, :slug, "parent-corp")

API Reference

Simple Lookup

# Returns :id field by default
Sow.lookup(schema, key, value)

# Example
country_id: Sow.lookup(MyApp.Country, :code, "NO")
# → Queries: SELECT id FROM countries WHERE code = 'NO'

Custom Field

# Return a different field
Sow.lookup(schema, key, value, field: :field_name)

# Example
country_name: Sow.lookup(MyApp.Country, :code, "NO", field: :name)
# → Queries: SELECT name FROM countries WHERE code = 'NO'

Multiple Match Criteria

# Match on multiple fields
Sow.lookup(schema, %{field1: value1, field2: value2})

# Example
org_id: Sow.lookup(MyApp.Organization, %{country_id: 1, type: "enterprise"})
# → Queries: SELECT id FROM organizations WHERE country_id = 1 AND type = 'enterprise'

Chained Lookups

Lookups can be nested to resolve values in order:

# First lookup country_id, then use it to find organization
org_id: Sow.lookup(MyApp.Organization, %{
  country_id: Sow.lookup(MyApp.Country, :code, "NO"),
  name: "ACME"
})

This resolves as:

1. Query countries for id where code = "NO" → returns 42
2. Query organizations for id where country_id = 42 AND name = "ACME"

Examples

E-commerce Fixtures

defmodule MyApp.Seeds.Orders do
  use Sow, schema: MyApp.Order, keys: [:reference]

  def records do
    [
      %{
        reference: "ORD-001",
        # Look up existing customer
        customer_id: Sow.lookup(MyApp.Customer, :email, "john@example.com"),
        # Look up product by SKU
        product_id: Sow.lookup(MyApp.Product, :sku, "WIDGET-PRO"),
        # Look up shipping zone
        shipping_zone_id: Sow.lookup(MyApp.ShippingZone, %{
          country_id: Sow.lookup(MyApp.Country, :code, "NO"),
          type: "standard"
        })
      }
    ]
  end
end

Multi-tenant Fixtures

defmodule MyApp.Seeds.TenantData do
  use Sow, schema: MyApp.Setting, keys: [:tenant_id, :key]

  def records do
    [
      %{
        tenant_id: Sow.lookup(MyApp.Tenant, :slug, "acme"),
        key: "theme",
        value: "dark"
      },
      %{
        tenant_id: Sow.lookup(MyApp.Tenant, :slug, "acme"),
        key: "timezone",
        value: "Europe/Oslo"
      }
    ]
  end
end

Using Lookup with Inline Records

defmodule MyApp.Seeds.Flows do
  use Sow, schema: MyApp.Flow, keys: [:slug]

  def records do
    [
      %{
        slug: "checkout",
        stages: Sow.has_many_inline(
          [
            %{
              position: 1,
              # Lookup stage that's managed elsewhere
              stage_id: Sow.lookup(MyApp.Stage, :type, "cart")
            },
            %{
              position: 2,
              stage_id: Sow.lookup(MyApp.Stage, :type, "payment")
            }
          ],
          schema: MyApp.FlowStage,
          foreign_key: :flow_id,
          keys: [:flow_id, :position]
        )
      }
    ]
  end
end

Error Handling

If a lookup fails to find a record, Sow raises at sync time:

** (Ecto.NoResultsError) expected at least one result but got none in query:

from c0 in MyApp.Country,
  where: c0.code == ^"XX"

Ensure the data exists before syncing fixtures that depend on it.

Comparison with belongs_to

# belongs_to: Syncs Countries fixture, then uses result
country: Sow.belongs_to(MyApp.Seeds.Countries, :code, "NO")

# lookup: Just queries database, no sync
country_id: Sow.lookup(MyApp.Country, :code, "NO")

Key differences:

• belongs_to creates a dependency in the sync graph
• lookup has no dependencies, queries at sync time
• belongs_to sets the association field (:country)
• lookup sets the foreign key field (:country_id)

Next Steps

• Create Wrapper Modules to share lookup helpers
• Learn about Associations for fixture-based references