diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1971af0..df58e86 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -21,6 +21,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Process incoming `Move()` activities in background.
 - Allow custom emojis with `image/webp` media type.
 
+### Fixed
+
+- Added missing `CHECK` constraints to database tables.
+
 ## [1.19.1] - 2023-03-31
 
 ### Changed
diff --git a/mitra-models/migrations/V0051__relationship__source_id_target_id_not_equal.sql b/mitra-models/migrations/V0051__relationship__source_id_target_id_not_equal.sql
new file mode 100644
index 0000000..4476f44
--- /dev/null
+++ b/mitra-models/migrations/V0051__relationship__source_id_target_id_not_equal.sql
@@ -0,0 +1,5 @@
+ALTER TABLE relationship ADD CONSTRAINT relationship_source_id_target_id_check CHECK (source_id != target_id);
+ALTER TABLE follow_request ADD CONSTRAINT follow_request_source_id_target_id_check CHECK (source_id != target_id);
+ALTER TABLE post_link ADD CONSTRAINT post_link_source_id_target_id_check CHECK (source_id != target_id);
+ALTER TABLE invoice ADD CONSTRAINT invoice_sender_id_recipient_id_check CHECK (sender_id != recipient_id);
+ALTER TABLE subscription ADD CONSTRAINT subscription_sender_id_recipient_id_check CHECK (sender_id != recipient_id);
diff --git a/mitra-models/migrations/schema.sql b/mitra-models/migrations/schema.sql
index 9141aae..8839db7 100644
--- a/mitra-models/migrations/schema.sql
+++ b/mitra-models/migrations/schema.sql
@@ -95,7 +95,8 @@ CREATE TABLE relationship (
     source_id UUID NOT NULL REFERENCES actor_profile (id) ON DELETE CASCADE,
     target_id UUID NOT NULL REFERENCES actor_profile (id) ON DELETE CASCADE,
     relationship_type SMALLINT NOT NULL,
-    UNIQUE (source_id, target_id, relationship_type)
+    UNIQUE (source_id, target_id, relationship_type),
+    CHECK (source_id != target_id)
 );
 
 CREATE TABLE follow_request (
@@ -104,7 +105,8 @@ CREATE TABLE follow_request (
     target_id UUID NOT NULL REFERENCES actor_profile (id) ON DELETE CASCADE,
     activity_id VARCHAR(250) UNIQUE,
     request_status SMALLINT NOT NULL,
-    UNIQUE (source_id, target_id)
+    UNIQUE (source_id, target_id),
+    CHECK (source_id != target_id)
 );
 
 CREATE TABLE post (
@@ -166,7 +168,8 @@ CREATE TABLE post_tag (
 CREATE TABLE post_link (
     source_id UUID NOT NULL REFERENCES post (id) ON DELETE CASCADE,
     target_id UUID NOT NULL REFERENCES post (id) ON DELETE CASCADE,
-    PRIMARY KEY (source_id, target_id)
+    PRIMARY KEY (source_id, target_id),
+    CHECK (source_id != target_id)
 );
 
 CREATE TABLE emoji (
@@ -219,7 +222,8 @@ CREATE TABLE invoice (
     amount BIGINT NOT NULL CHECK (amount >= 0),
     invoice_status SMALLINT NOT NULL DEFAULT 1,
     created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    UNIQUE (chain_id, payment_address)
+    UNIQUE (chain_id, payment_address),
+    CHECK (sender_id != recipient_id)
 );
 
 CREATE TABLE subscription (
@@ -230,5 +234,6 @@ CREATE TABLE subscription (
     chain_id VARCHAR(50) NOT NULL,
     expires_at TIMESTAMP WITH TIME ZONE NOT NULL,
     updated_at TIMESTAMP WITH TIME ZONE NOT NULL,
-    UNIQUE (sender_id, recipient_id)
+    UNIQUE (sender_id, recipient_id),
+    CHECK (sender_id != recipient_id)
 );