By name: ecto-thinking description: Apply when adding database tables, creating contexts, querying DB, adding schema fields, validating form input, fixing N+1 queries, preloading associations, or when the user mentions Repo, changesets, migrations, Ecto.Multi, has_many, belongs_to, transactions, query composition, or how contexts should talk.
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/1overRepo.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.