![]() ![]() ![]() Inside Ecto migrations, we can accomplish this by wrapping the command in an execute(). CREATE INDEX ON reviews((metadata->'item'->'price')) In a large table where a particularly embedded field is important enough to query, such as price in this case, it's a good idea to add an index. To find all reviews with such a structure and a price of 35, we can do the following: select * from metadata These can be queried in the DB with: select * from metadataĭigging in further we could examine only reviews with an an item field in the metadata and another nested field price. Over time, we might come to find that some reviews include an item field in their metadata. # How do we create an index a JSONB sub-field?Įnd Querying into JSONB fields with raw SQL It's particularly useful for situations involving user-submitted data, where we don't know the exact shape the data will come in.Ĭonsider this schema for user-submitted reviews: defmodule doĪdd :rating, :integer, default: 0, null: falseĬreate(unique_index(:reviews, )) Ecto has a map type, which is implemented in Postgres as a JSONB field. In this case, we could either build a Ecto.DateTime struct, or we could specify to Ecto that we’d like to cast the expression to Ecto.Date instead of Ecto.DateTime: Ectoing. This will, however, give us a Ecto.CastError, since an Ecto.Date struct cannot be cast to an Ecto.DateTime struct (since we’re comparing the interpolated Ecto.Date expressions to a field of type Ecto.DateTime). Let’s now take a look at a simple query that uses a join to fetch a user and their messages: SELECT * FROM users u INNER JOIN messages m ON u. For example: # Ectoing.Messageīelongs_to :user, Ectoing. This default behavior can be overridden by manually specifying the foreign key name by specifying the foreign_key option. By default, Ecto will append an _id onto the belongs_to association name and use that as the foreign key name (so here, it would be :user_id). We have also named the association as :user. Here, we’re saying that the model, Ectoing.Message, belongs to the Ectoing.User model. In the Ectoing.Message model, we define the following association relationship: belongs_to :user, Ectoing. We can see that one user has many messages ( Ectoing.Message), and we’re calling this association :messages. The schema defined in Ectoing.User defines the following association: has_many :messages, Ectoing. Looking at our Ectoing application, we can see one example of an association between the Ectoing.User model and the Ectoing.Message model. They are defined in the schemas for each model using the has_one/3 and has_many/3 macros (for models containing other models), and the belongs_to/3 macro (for models that are apart of other models - those that have the foreign keys). Simply put, associations enable developers to handle table relationships (implemented as foreign keys) in the models. Whilst these are not difficult to learn about, they’re not quite as trivial as the other topics covered so far. ![]() The reason for this is because learning about joins in Ecto alone is not useful: we need to know about associations as well. Joins are pretty fundamental when querying, and yet we’re only just covering them now. q oupby(name).avg(age) Select stats q lect(count(), avg(age), min(age). Each may also be used in conjunction with other queries to perform other application needs too, all without unnecessarily repeating parts of the query throughout the codebase. elixir use Qh Qh.configure(app: :myapp) q User.where(age >. It’s not hard to imagine that all of the above three queries could be used together to provide search results for when a particular user is searched for. The third query ( paginate_query) builds off of the second, where it limits the results and fetches them from a particular offset (to provide the basis for pagination). The second query ( search_by_username) builds off the first by filtering usernames according to some username we are searching for. The first query ( get_users_overview) is just a generic query to retrieve basic user information. The SQL version is quite repetitive, but the Ecto version on the other hand is quite DRY. | > limit ( 10 ) | > offset ( ^offset ) ) Ectoing. Paginate_query = from search_by_username , Search_by_username = from u in get_users_overview , Username = "%tp%" # Keywords query syntax Separate queries in Ecto can be combined together, allowing for reusable queries to be created.įor example, let’s see how we can create three separate queries and combine them together to achieve DRYer and more reusable code: SELECT id, username FROM users SELECT id, username FROM users WHERE username LIKE "%tp%" SELECT id, username FROM users WHERE username LIKE "%tp%" LIMIT 10, 0 offset = 0 ![]()
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