Ecto Thinking

Resumo (pt-BR): Contexto como bounded context (DDD). Entre contextos use IDs, não associations. Múltiplos changesets por schema; preload vs join; gotchas com CTE, pgbouncer, sandbox e null bytes.

Mental shifts for Ecto and data layer design. These insights challenge typical ORM patterns.

Context = Setting That Changes Meaning

Context isn't just a namespace—it changes what words mean. Think top-down: Subdomain → Context → Entity.

Cross-Context References: IDs, Not Associations

Use field :product_id, :integer in another context; query through the context, not across associations. Keeps contexts independent and testable.

DDD Patterns as Pipelines

Build → validate → insert. Use events (as data structs) to compose bounded contexts with minimal coupling.

Schema ≠ Database Table

Database table: standard schema/2
Form validation only: embedded_schema/1
API request/response: embedded schema or schemaless

Multiple Changesets per Schema

Different operations = different changesets (e.g. registration_changeset, profile_changeset, admin_changeset).

Multi-Tenancy

Composite foreign keys; use prepare_query/3 for automatic scoping. CTEs don't inherit schema prefix—set explicitly: %{recursive_query | prefix: "tenant"}.

Preload vs Join

Separate preloads: has-many with many records (less memory)
Join preloads: belongs-to, has-one (single query). Join preloads can use 10x more memory for has-many.

Gotchas

Parameterized ≠ prepared: pgbouncer → use prepare: :unnamed
pool_count vs pool_size: single larger pool often better for mixed workloads
Sandbox: Doesn't work with external processes (Cachex, GenServers)—they don't share the transaction
Null bytes: PostgreSQL rejects them; sanitize with String.replace(string, "\x00", "")
preload_order: Works for associations; not for through
Runtime migrations: Use list API with Ecto.Migrator.run/4

Idioms

Prefer Repo.insert/1 over Repo.insert!/1
Use Repo.transact/1 (Ecto 3.12+) for simple transactions instead of Ecto.Multi when appropriate

Red Flags - STOP and Reconsider

belongs_to pointing to another context's schema
Single changeset for all operations
Preloading has-many with join
CTEs in multi-tenant apps without explicit prefix
Using pgbouncer without prepare: :unnamed
Accepting user input without null byte sanitization

Any of these? Re-read the Gotchas section.

ecto-thinking