What are you doing?

Using the has[Association]() method on a Model instance

...Model defs...

ModelA.belongsToMany(models.ModelB, { through: ModelC });
ModelB.belongsToMany(models.ModelA, { through: ModelC });

...create entries...

instanceA.hasB(instanceB);

What do you expect to happen?

When a has[Association]() method is called that a lightweight (0 attributes) SELECT is made to confirm the association without wasting resources.

What is actually happening?

Calling has[Association]() makes a heavy INNER JOIN SELECT query.

• selects all attributes from the associated table and the join table
• none of the selected attributes are returned / used
• since the method returns a boolean true or false (confirmation method) this big SELECT is pointless

calling instanceA.hasB(instanceB) results in the following SQL echo:

Executing (default): SELECT "ModelB"."id", "ModelB"."email", "ModelB"."first_name", "ModelB"."last_name", "ModelB"."company", "ModelB"."created_at", "ModelB"."updated_at", "ModelC"."id" AS "ModelC.id", "ModelC"."paid" AS "ModelC.paid", "ModelC"."created_at" AS "ModelC.created_at", "ModelC"."updated_at" AS "ModelC.updated_at", "ModelC"."a_id" AS "ModelC.a_id", "ModelC"."b_id" AS "ModelC.b_id" FROM "bs" AS "ModelB" INNER JOIN "cs" AS "ModelC" ON "ModelB"."id" = "ModelC"."b_id" AND "ModelC"."a_id" = 50 WHERE ((("ModelB"."id" = 97)));

Temporary Solution

correct boolean result returned when modifying call as

• instanceA.hasB(instanceB, { attributes: [], joinTableAttributes: [] });
• now makes a lightweight SELECT:

Executing (default): SELECT  FROM "bs" AS "ModelB" INNER JOIN "cs" AS "ModelC" ON "ModelB"."id" = "ModelC"."b_id" AND "ModelC"."a_id" = 50 WHERE ((("ModelB"."id" = 97)));

Suggestion

Change internal behavior of has[Association]() method calls to use options object { attributes: [], joinTableAttributes: [] } so that this operation does not perform such a heavy query.

Output, either JSON or SQL

Dialect: postgres (but I believe any should be affected)
Dialect version: 9.6.10
Database version: 9.6.10
Sequelize version: 4.43.0
Tested with latest release: No (If yes, specify that version)

Note : Your issue may be ignored OR closed by maintainers if it's not tested against latest version OR does not follow issue template.