You're seeing just the function add, go back to Ecto.Migration module for more information.
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add(column, type, opts \\ [])

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Adds a column when creating or altering a table.

This function also accepts Ecto primitive types as column types that are normalized by the database adapter. For example, :string is converted to :varchar, :binary to :bits or :blob, and so on.

However, the column type is not always the same as the type used in your schema. For example, a schema that has a :string field can be supported by columns of type :char, :varchar, :text, and others. For this reason, this function also accepts :text and other type annotations that are native to the database. These are passed to the database as-is.

To sum up, the column type may be either an Ecto primitive type, which is normalized in cases where the database does not understand it, such as :string or :binary, or a database type which is passed as-is. Custom Ecto types like Ecto.UUID are not supported because they are application-level concerns and may not always map to the database.


create table("posts") do
  add :title, :string, default: "Untitled"

alter table("posts") do
  add :summary, :text # Database type
  add :object,  :map  # Elixir type which is handled by the database


  • :primary_key - when true, marks this field as the primary key. If multiple fields are marked, a composite primary key will be created.
  • :default - the column's default value. It can be a string, number, empty list, list of strings, list of numbers, or a fragment generated by fragment/1.
  • :null - when false, the column does not allow null values.
  • :size - the size of the type (for example, the number of characters). The default is no size, except for :string, which defaults to 255.
  • :precision - the precision for a numeric type. Required when :scale is specified.
  • :scale - the scale of a numeric type. Defaults to 0.
  • :after - positions field after the specified one. Only supported on MySQL, it is ignored by other databases.