Starting with Ecto 3.0, Ecto.Adapters.Postgres is not shipped with Ecto by default, therefore you have to add ecto_sql to the Mixfile dependencies:
###########
# mix.exs #
###########
defp deps do
[
# (...)
{:ecto_sql, "~> 3.0-rc.1"},
{:postgrex, ">= 0.0.0"}
]
end
# Feeling skittish about dependencies,
# I usually do this instead of simply
# doing `mix deps.get`:
$ mix deps.clean --all
$ mix do deps.get, compile
(The Ecto github repo v3.0.0 tree recommends {:ecto_sql, "~> 3.0"}, but the latest release is the 3.0.0-rc.1) therefore it won't work as of now. Interestingly there is no 3.0.0-rc.1 tag in the repo, but the documentation already refers to that and it also works with mix.)
...or, as Yufrend recommends in his answer, if you are starting a new Phoenix project, use < 1.4.0 packages.
See José Valim's “A sneak peek at Ecto 3.0” series where the first post explains the breaking changes in Ecto 3.0:
Split Ecto into ecto and ecto_sql
Ecto 3.0 will be broken in two repositories: ecto and ecto_sql.
Since Ecto 2.0, an increased number of developers and teams have been
using Ecto for data mapping and validation, without a need for a
database. However, adding Ecto to your application would still bring a
lot of the SQL baggage, such as adapters, sandboxes and migrations,
which many considered to be a mixed message.
In Ecto 3.0, we will move all of the SQL adapters to a separate
repository and Ecto will focus on the four building blocks: schemas,
changesets, queries and repos. You can see the discussion in the
issues tracker.
If you are using Ecto with a SQL database, migrating to Ecto 3.0 will
be very straight-forward. Instead of:
{:ecto, "~> 2.2"}
You should list:
{:ecto_sql, "~> 3.0"}
And if you are using Ecto only for data manipulation but with no
database access, then it is just a matter of bumping its version.
That’s it!
UPDATE
For some reason, I also needed to add {:plug_cowboy, "~> 1.0"} to the Mixfile dependencies when updating a Phoenix 1.3 project and it all started working.
mix.exsfile. Do you have something like{:postgrex, ">= 0.0.0"}in your dependencies? – Justin Woodecto_sqlas a dependency? – Justin Wood