Make a previously existing foreign key column have a unique constraint in postgres - postgresql

Question

I need to create a migration for an already existing table to make it's foreign key have a UNIQUE constraint. How do I do this?
From the examples I found in the documentation, it is mostly done when the table is created. The issue is I need to add this onto a column that already exists and is already set as a foreign key. This is what the table looks like at it's creation:
CREATE TABLE IF NOT EXISTS "myTable" (
"_id" SERIAL NOT NULL,
"myForeignKeyId" INTEGER NOT NULL,
"name" VARCHAR(255) NOT NULL,
CONSTRAINT "pk_myTable" PRIMARY KEY ("_id"),
CONSTRAINT "fk_myTable_myForeignKeyId" FOREIGN KEY ("myForeignKeyId") REFERENCES "myOtherTable" ("_id")
);
What I want to do is on a migration make myForeignKeyId unique. How do I do that?
I have tried to following:
CREATE UNIQUE INDEX CONCURRENTLY "myTable_myForeignKeyId"
ON province ("myForeignKeyId");
ALTER TABLE IF EXISTS "myTable"
ADD CONSTRAINT "myForeignKeyId"
UNIQUE USING INDEX "myTable_myForeignKeyId";
First off, when I try this in a migration I get the error:
CREATE INDEX CONCURRENTLY cannot run inside a transaction block
So that part cannot be done, but even just doing it through SQL, the second part doesn't work either as it claims myForeignKeyId already exists. Even if I add an ALTER COLUMN myForeignKeyId it just says there is an error on that line.
This seems like it should be a simple enough operation, how can I do this?

Answer 1

After digging some more found quite a simple way to do this, was clearly originally off target.
To add a unique constraint to a column:
ALTER TABLE "myTable"
ADD CONSTRAINT "myUniqueKeyNameOfChoice" UNIQUE ("myColumn");
To remove it:
ALTER TABLE "myTable"
DROP CONSTRAINT "myUniqueKeyNameOfChoice";