Can Ecto schema fields be marked private to avoid accidental JSON exposure?

No, Ecto does not provide a built-in 'private' or 'public' attribute for schema fields to control automatic JSON serialization. In Elixir, a schema is essentially a wrapper around a map; any JSON encoder (such as Jason or Poison) will serialize all keys present in the struct unless explicitly instructed to do otherwise. Because visibility is not enforced at the schema or repository level, the idiomatic pattern for consistent field exclusion is to delegate output shaping to a dedicated layer rather than relying on the schema itself. ### Idiomatic Patterns for Field Exclusion To ensure sensitive fields are never leaked accidentally, consider the following strategies:

• Explicit View Modules (Recommended): Instead of passing the schema directly to the renderer, use a dedicated module (often called a JSONView or a similar pattern) that explicitly defines an allow-list. This ensures that any new fields added to the database are not exposed until they are manually added to the view.
• Custom JSON Serialization with Options: If you use `Jason.encode/2`, you can pass `:only` or `:except` keys. However, using `:except` is considered a "deny-list" approach, which is riskier because new fields added to the schema later will be included by default.
• Virtual Fields for Volatile Data: While virtual fields are not persisted to the database, they still exist in the struct. If you use virtual fields for temporary calculations, you must still explicitly filter them out during the serialization process.

### Practical Verification To verify how your current schema handles serialization, you can run a simple check in IEx:

# Assume a User schema with a :password_hash field
user = Repo.get(User, 1)
# This will include password_hash in the output
Jason.encode(user)

To fix this consistently, implement a projection function in your view layer:

# In your controller or view module
def to_json(user) do
  %{
    id: user.id,
    email: user.email,
    updated_at: user.updated_at
  } # password_hash is explicitly omitted
end

Are you currently using a specific web framework like Phoenix, or are you building a custom API implementation? This would determine the best specific view pattern to use.