ecto-thinking

# Ecto Thinking 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. "Product" means different things in Checkout (SKU, name), Billing (SKU, cost), and Fulfillment (SKU, warehouse). Each bounded context may have its OWN Product schema/table. **Think top-down:** Subdomain → Context → Entity. Not "What context does Product belong to?" but "What is a Product in this business domain?" ## Cross-Context References: IDs, Not Associations ```elixir schema "cart_items" do field :product_id, :integer # Reference by ID # NOT: belongs_to :product, Catalog.Product end ``` Query through the context, not across associations. Keeps contexts independent and testable. ## DDD Patterns as Pipelines ```elixir def create_product(params) do params |> Products.build() # Factory: unstructured → domain |> Products.validate() # Aggregate: enforce invariants |> Products.insert() # Repository: persist end ``` Use events (as data structs) to compose bounded contexts with minimal coupling. ## Schema ≠ Database Table | Use Case | Approach | |----------|----------| | Database table | Standard `schema/2` | | Form validation only | `embedded_schema/1` | | API request/response | Embedded schema or schemaless | ## Multiple Changesets per Schema ```elixir def registration_changeset(user, attrs) # Full validation + password def profile_changeset(user, attrs) # Name, bio only def admin_changeset(user, attrs) # Role, verified_at ``` Different operations = different changesets. ## Multi-Tenancy: Composite Foreign Keys ```elixir add :post_id, references(:posts, with: [org_id: :org_id], match: :full) ``` Use `prepare_query/3` for automatic scoping. Raise if `org_id` missing. ## Preload vs Join Trade-offs | Approach | Best For | |----------|----------| | 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. ## CRUD Contexts Are Fine > "If you have a CRUD bounded context, go for it. No need to add complexity." Use generators for simple cases. Add DDD patterns only when business logic demands it. ## Gotchas from Core Team ### CTE Queries Don't Inherit Schema Prefix In multi-tenant apps, CTEs don't get the parent query's prefix. **Fix:** Explicitly set prefix: `%{recursive_query | prefix: "tenant"}` ### Parameterized Queries ≠ Prepared Statements - **Parameterized queries:** `WHERE id = $1` — always used by Ecto - **Prepared statements:** Query plan cached by name — can be disabled **pgbouncer:** Use `prepare: :unnamed` (disables prepared statements, keeps parameterized queries). ### pool_count vs pool_size More pools with fewer connections = better for benchmarks. **But** with mixed fast/slow queries, a single larger pool gives better latency. **Rule:** `pool_count` for uniform workloads, larger `pool_size` for real apps. ### Sandbox Mode Doesn't Work With External Processes Cachex, separate GenServers, or anything outside the test process won't share the sandbox transaction. **Fix:** Make the external service use the test process, or accept it's not in the same transaction. ### Null Bytes Crash Postgres PostgreSQL rejects null bytes even though they're valid UTF-8. **Fix:** Sanitize at boundaries: `String.replace(string, "\x00", "")` ### preload_order for Association Sorting ```elixir has_many :comments, Comment, preload_order: [desc: :inserted_at] ``` Note: Doesn't work for `through` associations. ### Runtime Migrations Use List API ```elixir Ecto.Migrator.run(Repo, [{0, Migration1}, {1, Migration2}], :up, opts) ``` ## Idioms - Prefer `Repo.insert/1` over `Repo.insert!/1`—handle `{:ok, _}` / `{:error, _}` explicitly - Use `Repo.transact/1` (Ecto 3.12+) for simple transactions instead of `Ecto.Multi` ## 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` - Testing with Cachex/GenServers assuming sandbox shares transactions - Accepting user input without null byte sanitization **Any of these? Re-read the Gotchas section.**